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

[Nikita Popov via cfe-commits]

nikic wrote:

https://github.com/llvm/llvm-project/pull/78114 should absolutely not go onto the release branch -- and with that in mind, please hold off on merging this PR until LLVM 19 has branched as well.

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