Choose and document minimum supported Python version

[randombit]

Currently Botan's minimum supported Python version is undocumented and effectively implicit on what language features we use.

Since 3.0.0 we do require at least Python3 of some kind.

The oldest version we test with in CI is Python 3.10, from Ubuntu 22.04.

The oldest non-EOL version of Python is Python 3.9.

Possible options

1. We support Python 3.10 and higher (don't claim to support what we aren't testing)
2. We support all non-EOL versions of Python, and each year as Python.org EOLs a new release we drop support for it.
3. We declare some specific older version (eg 3.5 or 3.8 ...) our MSPV, and ideally test it

(1) might leave some users excluded

(2) is problematic as it conflicts with the general policy that the toolchain minimum versions do not bump in the course of a major release.

(3) might be hard to test in practice, and if we're not testing it, it is somewhat silly to claim we support it.