`libcst.tool`: add support Python 3.9, 3.10, 3.11, 3.12 and 3.13

[jorenham]

Currently, running e.g. libcst.tool codemod --python-version=3.9 ... will raise a ValueError with the message:

ValueError: LibCST can only parse code using one of the following versions of Python's grammar: 3.0, 3.1, 3.3, 3.5, 3.6, 3.7, 3.8. More versions may be supported by future releases.

This adds the missing supported python version strings.

[zsol]

Unfortunately that flag doesn't actually do anything anymore by default[^1], so I'd rather remove it.

[^1]: It only controls the behavior of the old, unmaintained pure python parser which you can enable with an environment variable. That parser doesn't support 3.9 or newer.