sysvorOrt /home/michi # emerge -avuND world

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

Calculating dependencies \

!!! Problem resolving dependencies for dev-tcltk/tclpython from @selected
... done!

!!! The ebuild selected to satisfy "dev-tcltk/tclpython" has unmet requirements.
- dev-tcltk/tclpython-5.0::gentoo USE="" ABI_X86="(64)" PYTHON_SINGLE_TARGET="-python3_6 -python3_7"

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

(dependency required by "@selected" [set])
(dependency required by "@world" [argument])
sysvorOrt /home/michi # 

Um dieses Problem zu beheben ist ein Eintrag in der /etc/portage/package.use erforderlich:

dev-tcltk/tclpython python_single_target_python3_7 

Die Version von Python (3_7) kann natürlich angepasst werden falls gewünscht.