PEP 838: Add `python-version` to `pyvenv.cfg`

[konstin]

I'm preparing draft PRs to accompany the (as of writing draft) PEP 838 as reference implementations. The PRs are blocked on a decision on the PEP itself.

Linked PRs

• gh-154377: Add python-version to pyvenv.cfg (PEP 838) #154378
• gh-154377: Reject startup with mismatched python-version (PEP 838) #154381

[picnixz]

This is not the way we accept PRs for PEPs. PRs for PEPs should be opened on forks instead of the main repository. The PEP has not been discussed. Please wait for the PEP to be accepted. We accept PRs against the main repo if buildbots or other infrastructure is needed but this one does not seem to be the case.