[clang] [libclang/python] Change all global variables to snake case (PR #132378)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

I was pointed out that there is a separate rule that constants are supposed to be in `SCREAMING_CAPS` (https://peps.python.org/pep-0008/#constants). We should've changed the names (`k` prefix is out of place anyway), but not to `snake_case` as I implied.

https://github.com/llvm/llvm-project/pull/132378