Python version support policy

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.

没有指定文件或测试。首先定位说明 Python 版本支持情况的仓库文档和 CI 配置，然后将它们与拟议的策略进行比较。当受支持版本策略得到清晰记录，并且与项目声明的 CI 覆盖范围一致时，即视为完成。