[clang] [libclang/python] Fix bugs in custom enum implementation and add tests (PR #95381)

[Jannick Kremer via cfe-commits]

DeinAlptraum wrote:

@Endilll I merged the Python 3.8 CI into this PR and the CI run was successful (though it only ran on 3.8 for some reason), so can this be merged?

https://github.com/llvm/llvm-project/pull/95381