[clang/python] check-clang-python currently fails due to outdated Python version

Since upgrading the minimum Python version recently, https://github.com/llvm/llvm-project/commit/33f4a77d9218c1a2d5c994a8a8398479731612dc the `check-clang-python` test for the Clang-Python-Bindings are failing (see https://github.com/llvm/llvm-project/actions/workflows/libclang-python-tests.yml) as those are still testing for Python 3.7. This should be upgraded to the new minimum version of 3.8.