[clang] [libclang/python] Fix visitor callback return data type (PR #221024)

[Peter Bierma via cfe-commits]

ZeroIntensity wrote:

I've merged a fix for this upstream, which will be available in Python 3.13.16 and 3.14.8 sometime in the next week or so (see PEPs [719](https://peps.python.org/pep-0719/) and [745](https://peps.python.org/pep-0745/) for the release schedule).

https://github.com/llvm/llvm-project/pull/221024