Make ConfigSetting generic

[septatrix]

This prevents one from accidentally using incompatible parse/match/default_factory callbacks. A few type ignores are necessary due to python/mypy#3737 and in finalize_config because any inference is impossible there

[behrmann]

I really like this! I was holding off from adding generics, since this has changed so much from our lower bar of 3.9 to now, that I didn't think it was worth it, but looks like it already works somewhat sufficiently. Nice!

[behrmann]

One thing I forgot: This should probably be checked on Python 3.9 to ensure this doesn't introduce anything by chance that fails on 3.9.

[septatrix]

One thing I forgot: This should probably be checked on Python 3.9 to ensure this doesn't introduce anything by chance that fails on 3.9.

Mypy and pyright both assume Python 3.9 in CI

[behrmann]

Mypy and pyright both assume Python 3.9 in CI

Yeah, but type checking and actually running without throwing an exception (e.g. just the help output), are two different things, unfortunately. We've had plenty of cases where successfully type-checked code crashed because of circular imports right at import time. :)