[clang] [libclang/python] Fix some type errors, add type annotations (PR #98745)

[Jannick Kremer via cfe-commits]

DeinAlptraum wrote:

I added breaking change notes where I thought them appropriate, could you merge if you are happy with this?

Regarding the relase: I believe I've taken care not to break anything (besides things noted in breaking change notes) so hopefully not much need for that. If any unexpected breakages do come to my attention, I will try to get that in though.
This PR is also probably the last step before #78114, so that one can hopefully still be picked onto `releases/19.x` so that we have the entire "strict typing" thing in the release.

https://github.com/llvm/llvm-project/pull/98745