I have instaled version 3.0 of rhythmbox, wich has been replaced for version 3.0-r1 in portage tree.
Here is the portage message:

>>> Emerging (1 of 1) media-sound/rhythmbox-3.0-r1

• rhythmbox-3.0.tar.xz SHA256 SHA512 WHIRLPOOL size ... [ ok ]
• No Python implementation selected for the build. Please set
• the PYTHON_SINGLE_TARGET variable in your make.conf to one
• of the following values:
• python3_2

• ERROR: media-sound/rhythmbox-3.0-r1 failed (setup phase):
• No supported Python implementation in PYTHON_SINGLE_TARGET.
• Call stack:
• ebuild.sh, line 93: Called pkg_setup
• rhythmbox-3.0-r1.ebuild, line 102: Called python-single-r1_pkg_setup
• python-single-r1.eclass, line 250: Called die
• The specific snippet of code:
• die "No supported Python implementation in PYTHON_SINGLE_TARGET."
• If you need support, post the output of `emerge --info '=media-sound/rhythmbox-3.0-r1'`,
• the complete build log and the output of `emerge -pqv '=media-sound/rhythmbox-3.0-r1'`.
• The complete build log is located at '/var/tmp/portage/media-sound/rhythmbox-3.0-r1/temp/build.log'.
• The ebuild environment file is located at '/var/tmp/portage/media-sound/rhythmbox-3.0-r1/temp/die.env'.
• Working directory: '/usr/lib64/portage/pym'
• S: '/var/tmp/portage/media-sound/rhythmbox-3.0-r1/work/rhythmbox-3.0'