Document the supported stdlib versions

[srittau]

Closes: #12716

[AlexWaygood]

Maybe something like "We will cease actively testing support for a Python version"? The stubs for that version will probably continue to be entirely accurate in the three months when the version is untested but the branches are still there

[srittau]

We do more than that, though. We will declare end of support in the docs. I don't think we need to be completely accurate, especially since this policy is mostly relevant to typeshed and type checker developers. End users are only indirectly affected through their type checkers.

[AlexWaygood]

Fair enough. I'd prefer to be precise here since these CONTRIBUTING docs are already not exactly concise, so I don't think the marginal impact of another sentence is particularly big! But I don't have a strong opinion.

[JelleZijlstra]

Agree that it's useful to be precise about our versioning policy, so we don't have to wrangle over it again next year when we start to think about dropping 3.9.

[srittau]

My comment was contradictory anyway. Precision is good, but I want to word it in a way that makes it clear that official support is dropped after 2-3 months.

[AlexWaygood]

Thanks!

[hauntsaninja]

I suggested fixed numbers. I think a range isn't too helpful (in practice people always end up aiming to do either the lower bound or the upper bound, and may as well be explicit)