dev-python/matplotlib depends on the ebuild dev-python/versioneer, but that ebuild does not exist:

$ emerge -a matplotlib

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

Calculating dependencies... done!

emerge: there are no ebuilds to satisfy "dev-python/versioneer[python_targets_python3_10(-)?,python_targets_pypy3(-)?,python_targets_python3_7(-)?,python_targets_python3_9(-)?,python_targets_python3_8(-)?,-python_single_target_python3_10(-),-python_single_target_pypy3(-),-python_single_target_python3_7(-),-python_single_target_python3_9(-),-python_single_target_python3_8(-)]".
(dependency required by "dev-python/matplotlib-3.5.3::python-modules-kit" [ebuild])
(dependency required by "matplotlib" [argument])