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.

No se nombra ningún archivo ni prueba. Empieza por localizar la documentación del repositorio y la configuración de CI que describen la compatibilidad con las versiones de Python y, después, compáralas con la política propuesta. Se considera terminado cuando la política de versiones compatibles está documentada claramente y coincide con la cobertura de CI declarada por el proyecto.