$ emerge --ask --update --deep --newuse --verbose --keep-going @world

These are the packages that would be merged, in order:

Calculating dependencies... done!

!!! The ebuild selected to satisfy ">=dev-util/gtk-doc-1.3" has unmet requirements.

• dev-util/gtk-doc-1.25-r1::gnome-kit USE="-debug -doc -emacs -highlight -vim" PYTHON_SINGLE_TARGET="python2_7 python3_6 -python3_4 -python3_5 -python3_7" PYTHON_TARGETS="python2_7 python3_6 -python3_4 -python3_5 -python3_7"

The following REQUIRED_USE flag constraints are unsatisfied:
exactly-one-of ( python_single_target_python2_7 python_single_target_python3_7 python_single_target_python3_4 python_single_target_python3_5 python_single_target_python3_6 )

The above constraints are a subset of the following complete expression:
exactly-one-of ( python_single_target_python2_7 python_single_target_python3_7 python_single_target_python3_4 python_single_target_python3_5 python_single_target_python3_6 ) python_single_target_python2_7? ( python_targets_python2_7 ) python_single_target_python3_7? ( python_targets_python3_7 ) python_single_target_python3_4? ( python_targets_python3_4 ) python_single_target_python3_5? ( python_targets_python3_5 ) python_single_target_python3_6? ( python_targets_python3_6 )

(dependency required by "sys-fs/udisks-2.7.4-r1::gnome-kit" [installed])
(dependency required by "@selected" [set])
(dependency required by "@world" [argument])

$ emerge --info
Portage 2.3.47 (python 2.7.15-final-0, funtoo/1.0/linux-gnu/arch/x86-64bit, gcc-7.3.1, glibc-2.26-r7, 4.19.6 x86_64)
=================================================================
System uname: Linux-4.19.6-x86_64-Intel-R-Core-TM-_i7-6820HK_CPU@_2.70GHz-with-gentoo-2.2.2
KiB Mem: 65902252 total, 20717956 free
KiB Swap: 0 total, 0 free
sh bash 4.4_p18
ld GNU ld (Gentoo 2.29.1 p3) 2.29.1
ccache version 3.3.4 [enabled]
app-shells/bash: 4.4_p18::core-kit
dev-java/java-config: 2.2.0-r3::java-kit
dev-lang/perl: 5.24.0-r4::perl-kit
dev-lang/python: 2.7.15::python-kit, 3.6.5-r1::python-kit
dev-util/ccache: 3.3.4-r1::core-kit
dev-util/cmake: 3.10.1::core-kit
sys-apps/baselayout: 2.2.2::core-kit
sys-apps/openrc: 0.35.5-r1::core-kit
sys-apps/sandbox: 2.12::core-kit
sys-devel/autoconf: 2.13::core-kit, 2.69-r4::core-kit
sys-devel/automake: 1.15.1-r1::core-kit
sys-devel/binutils: 2.29.1-r2::core-kit
sys-devel/gcc: 7.3.1-r5::core-kit
sys-devel/gcc-config: 1.9.1::core-kit
sys-devel/libtool: 2.4.6-r4::core-kit
sys-devel/make: 4.2.1-r1::core-kit
sys-kernel/linux-headers: 4.14::core-kit (virtual/os-headers)
sys-libs/glibc: 2.26-r7::core-kit
Repositories:

nokit
location: /var/git/meta-repo/kits/nokit
masters: core-kit
priority: -500

livecd
location: /var/git/overlay/local
masters: core-kit
priority: 0

core-hw-kit
location: /var/git/meta-repo/kits/core-hw-kit
masters: core-kit
priority: 1

core-kit
location: /var/git/meta-repo/kits/core-kit
masters: core-kit
priority: 1
aliases: gentoo

desktop-kit
location: /var/git/meta-repo/kits/desktop-kit
masters: core-kit
priority: 1

dev-kit
location: /var/git/meta-repo/kits/dev-kit
masters: core-kit
priority: 1

editors-kit
location: /var/git/meta-repo/kits/editors-kit
masters: core-kit
priority: 1

games-kit
location: /var/git/meta-repo/kits/games-kit
masters: core-kit
priority: 1

gnome-kit
location: /var/git/meta-repo/kits/gnome-kit
masters: core-kit
priority: 1

haskell-kit
location: /var/git/meta-repo/kits/haskell-kit
masters: core-kit
priority: 1

java-kit
location: /var/git/meta-repo/kits/java-kit
masters: core-kit
priority: 1

kde-kit
location: /var/git/meta-repo/kits/kde-kit
masters: core-kit
priority: 1

lang-kit
location: /var/git/meta-repo/kits/lang-kit
masters: core-kit
priority: 1

lisp-scheme-kit
location: /var/git/meta-repo/kits/lisp-scheme-kit
masters: core-kit
priority: 1

llvm-kit
location: /var/git/meta-repo/kits/llvm-kit
masters: core-kit
priority: 1

media-kit
location: /var/git/meta-repo/kits/media-kit
masters: core-kit
priority: 1

ml-lang-kit
location: /var/git/meta-repo/kits/ml-lang-kit
masters: core-kit
priority: 1

net-kit
location: /var/git/meta-repo/kits/net-kit
masters: core-kit
priority: 1

perl-kit
location: /var/git/meta-repo/kits/perl-kit
masters: core-kit
priority: 1

php-kit
location: /var/git/meta-repo/kits/php-kit
masters: core-kit
priority: 1

python-kit
location: /var/git/meta-repo/kits/python-kit
masters: core-kit
priority: 1

python-modules-kit
location: /var/git/meta-repo/kits/python-modules-kit
masters: core-kit
priority: 1

ruby-kit
location: /var/git/meta-repo/kits/ruby-kit
masters: core-kit
priority: 1

science-kit
location: /var/git/meta-repo/kits/science-kit
masters: core-kit
priority: 1

security-kit
location: /var/git/meta-repo/kits/security-kit
masters: core-kit
priority: 1

text-kit
location: /var/git/meta-repo/kits/text-kit
masters: core-kit
priority: 1

xfce-kit
location: /var/git/meta-repo/kits/xfce-kit
masters: core-kit
priority: 1

xorg-kit
location: /var/git/meta-repo/kits/xorg-kit
masters: core-kit
priority: 1

gridcoin
location: /var/lib/layman/gridcoin
masters: core-kit
priority: 50

ACCEPT_KEYWORDS="amd64 ~amd64"
ACCEPT_LICENSE="*"
CBUILD="x86_64-pc-linux-gnu"
CFLAGS="-march=native -mtune=native -O2 -pipe"
CHOST="x86_64-pc-linux-gnu"
CONFIG_PROTECT="/etc /usr/lib64/libreoffice/program/sofficerc /usr/share/gnupg/qualified.txt"
CONFIG_PROTECT_MASK="/etc/ca-certificates.conf /etc/dconf /etc/env.d /etc/fonts/fonts.conf /etc/gconf /etc/gentoo-release /etc/revdep-rebuild /etc/sandbox.d /etc/terminfo"
CXXFLAGS="-march=native -mtune=native -O2 -pipe"
DISTDIR="/var/cache/portage/distfiles"
EMERGE_DEFAULT_OPTS="--jobs=8 --load-average=8 --keep-going --with-bdeps=y"
FEATURES="assume-digests binpkg-logs buildpkg ccache config-protect-if-modified distlocks ebuild-locks fixlafiles merge-sync multilib-strict news parallel-fetch preserve-libs protect-owned sandbox sfperms strict unknown-features-warn unmerge-logs unmerge-orphans userfetch userpriv usersandbox usersync xattr"
FFLAGS="-march=broadwell -O2 -pipe"
GENTOO_MIRRORS="https://fastpull-us.funtoo.org"
INSTALL_MASK="/etc/systemd/ /usr/lib/systemd/ /lib/systemd/"
LANG="en_US.utf8"
LDFLAGS="Wl,-O1 -Wl,sort-common -Wl,-as-needed"
LINGUAS="en_US fa_IR en fa"
MAKEOPTS="-j9 -l8"
PKGDIR="/var/cache/portage/packages"
PORTAGE_CONFIGROOT="/"
PORTAGE_TMPDIR="/var/tmp"
USE="X a52 aac acl alsa amd64 apng berkdb bluray bzip2 cdda cddb cdio cdr consolekit cracklib crypt cups curl cxx dbus dri dts dvd dvdr dvdread encode exif faac faad ffmpeg flac gdbm gif gpm ico iconv icu ieee1394 ios ipod ipv6 jpeg jpeg2k lame libass libguess libmpeg2 mad matroska mjpeg mmx modules mp3 mpeg mtp mudflap multilib ncurses nls nptl nsplugin ogg opengl openmp pam pcre pdf png postproc pppd python quicktime readline resolvconf sndfile sse sse2 ssl svg taglib tcpd theora tiff truetype twolame udev unicode v4l vorbis vpx wav wavpack webp win32codecs wmf x264 x265 xattr xml xvid zlib" ABI_X86="64" ALSA_CARDS="hda-intel" ALSA_PCM_PLUGINS="adpcm alaw asym copy dmix dshare dsnoop empty extplug file hooks iec958 ioplug ladspa lfloat linear meter mmap_emul mulaw multi null plug rate route share shm softvol" APACHE2_MODULES="actions alias auth_basic authn_alias authn_anon authn_dbm authn_default authn_file authz_dbm authz_default authz_groupfile authz_host authz_owner authz_user autoindex cache cgi cgid dav dav_fs dav_lock deflate dir disk_cache env expires ext_filter file_cache filter headers include info log_config logio mem_cache mime mime_magic negotiation rewrite setenvif speling status unique_id userdir usertrack vhost_alias authn_core authz_core socache_shmcb unixd" CALLIGRA_FEATURES="kexi words flow plan sheets stage tables krita karbon braindump author" COLLECTD_PLUGINS="df interface irq load memory rrdtool swap syslog" CPU_FLAGS_X86="aes avx avx2 f16c fma3 mmx mmxext pclmul popcnt sse sse2 sse3 sse4_1 sse4_2 ssse3" ELIBC="glibc" GPSD_PROTOCOLS="ashtech aivdm earthmate evermore fv18 garmin garmintxt gpsclock isync itrax mtk3301 nmea ntrip navcom oceanserver oldstyle oncore rtcm104v2 rtcm104v3 sirf skytraq superstar2 timing tsip tripmate tnt ublox ubx" GRUB_PLATFORMS="efi-64 pc" INPUT_DEVICES="joystick keyboard libinput mouse" KERNEL="linux" LCD_DEVICES="bayrad cfontz cfontz633 glk hd44780 lb216 lcdm001 mtxorb ncurses text" LIBREOFFICE_EXTENSIONS="presenter-console presenter-minimizer" LLVM_TARGETS="BPF NVPTX X86" OFFICE_IMPLEMENTATION="libreoffice" PHP_TARGETS="php5-6 php7-0" POSTGRES_TARGETS="postgres9_5" PYTHON_SINGLE_TARGET="python3_6" PYTHON_TARGETS="python3_6 python2_7" QEMU_SOFTMMU_TARGETS="i386 x86_64" QEMU_USER_TARGETS="i386 x86_64" RUBY_TARGETS="ruby23 ruby24" USERLAND="GNU" VIDEO_CARDS="nvidia v4l" XTABLES_ADDONS="quota2 psd pknock lscan length2 ipv4options ipset ipp2p iface geoip fuzzy condition tee tarpit sysrq steal rawnat logmark ipmark dhcpmac delude chaos account"
Unset: CPPFLAGS, CTARGET, ENV_UNSET, LC_ALL, PORTAGE_BINHOST, PORTAGE_BUNZIP2_COMMAND, PORTAGE_COMPRESS, PORTAGE_COMPRESS_FLAGS