Python version support policy

[JelleZijlstra]

I think it would be useful to have an explicit Python version support policy.

Here's a few aspects I'd like to propose:

• We support all feature versions (3.x) of Python from the the beginning of the release candidate phase until they reach end-of-life.
• We attempt to support pre-releases before the RC phase too, especially while they're in beta, but there may be some mismatches.
• We drop support for old versions sometime after they reach EOL, usually not immediately but only when CI maintenance starts to become a burden.
• We aim to support all bugfix releases (3.x.y) of supported feature versions. However, to keep CI sane we don't necessarily test against all possible bugfix releases.