[llvm] [libclang/python] Change minimum Python test version to 3.8 (PR #95210)

[Vlad Serebrennikov via llvm-commits]

Endilll wrote:

Despite status not showing it, both Python 3.8 and 3.11 runs finished successfully across two different runs:
https://github.com/llvm/llvm-project/actions/runs/9479974893/attempts/2?pr=95210
https://github.com/llvm/llvm-project/actions/runs/9479974893/attempts/3?pr=95210

https://github.com/llvm/llvm-project/pull/95210