python bindings: fix DeprecationWarning

Fixes observed warning running `ninja check-all`:
llvm-project/clang/bindings/python/tests/cindex/test_diagnostics.py:100:
DeprecationWarning: Please use assertRegex instead.
  self.assertRegexpMatches(children[0].spelling

Looks like unittest.assertRegexpMatches has been deprecated in favor of
unittest.assertRegex since Python 3.2, according to:
https://docs.python.org/3/library/unittest.html#deprecated-aliases

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D85692