[libclang/python] Refactor enum usage (llvm#95608)

Use Python's builtin enum class instead of writing our own.

This is preparation for passing a strict type check in PR llvm#78114 ,
fixing 927 out of 1341 strict typing errors

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Co-authored-by: Jannick Kremer <[email protected]>
Co-authored-by: Vlad Serebrennikov <[email protected]>

Author: 2 people