Any attempt I make to install an application or update the system ends like this:

********* /home/vienswuer # emerge --oneshot sys-apps/portage
/usr/lib64/python3.6/site-packages/portage/package/ebuild/_config/KeywordsManager.py:70: UserWarning: /etc/portage/package.keywords is deprecated, use /etc/portage/package.accept_keywords instead
UserWarning)

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Calculating dependencies \

!!! Problem resolving dependencies for sys-apps/portage
... done!

<pre>!!! Problem resolving dependencies for sys-apps/portage
... done!

!!! <font color="#FF5555"><b>The ebuild selected to satisfy </b></font><font color="#00AA00">&quot;sys-apps/portage&quot;</font><font color="#FF5555"><b> has unmet requirements.</b></font>
- sys-apps/portage-2.3.101-r2::gentoo USE=&quot;(<font color="#FF5555"><b>ipc</b></font>) <font color="#FF5555"><b>native-extensions</b></font> <font color="#FF5555"><b>rsync-verify</b></font> <font color="#FF5555"><b>xattr</b></font> <font color="#5555FF"><b>-apidoc</b></font> <font color="#5555FF"><b>-build</b></font> <font color="#5555FF"><b>-doc</b></font> <font color="#5555FF"><b>-gentoo-dev</b></font> (<font color="#5555FF"><b>-selinux</b></font>)&quot; ABI_X86=&quot;(<font color="#FF5555"><b>64</b></font>)&quot; PYTHON_TARGETS=&quot;<font color="#5555FF"><b>-pypy3</b></font> <font color="#5555FF"><b>-python3_6</b></font> <font color="#5555FF"><b>-python3_7</b></font> <font color="#5555FF"><b>-python3_8</b></font> <font color="#5555FF"><b>-python3_9</b></font>&quot;

The following REQUIRED_USE flag constraints are unsatisfied:
any-of ( python_targets_pypy3 python_targets_python3_6 python_targets_python3_7 python_targets_python3_8 python_targets_python3_9 )

</pre>


other useful info:

********* /home/vienswuer # eselect python list
Available Python interpreters, in order of preference:
[1] python3.5
[2] python2.7
[3] python3.6
********* /home/vienswuer # python
Python 3.5.5 (default, Aug 22 2018, 19:44:59)
[GCC 6.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

********* /home/vienswuer # emerge --ask --changed-use --deep @world

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

Calculating dependencies \

!!! Problem resolving dependencies for dev-libs/libnatspec:0 from @selected
... done!

!!! The ebuild selected to satisfy "dev-libs/libnatspec:0" has unmet requirements.
- dev-libs/libnatspec-0.3.0::gentoo USE="python -doc" ABI_X86="(64)" PYTHON_SINGLE_TARGET="-python2_7"

The following REQUIRED_USE flag constraints are unsatisfied:
python? ( python_single_target_python2_7 )

The above constraints are a subset of the following complete expression:
python? ( exactly-one-of ( python_single_target_python2_7 ) )

(dependency required by "@selected" [set])
(dependency required by "@world" [argument])

The following link provided me with the appropriate solution:

https://www.reddit.com/r/Gentoo/comm...t_propagating/

The file "use.stable.mask" did not exist, so it was created. Right after that, I could upgrade my system without problem.

echo '-python_targets_python3_7\n-python_single_target_python3_7' >> /etc/portage/profile/use.stable.mask