Fix unittest warning

[adamchainz]

Fix this warning:

/.../parameterized/test.py:38: DeprecationWarning: Please use assertRaisesRegex instead.
  with self.assertRaisesRegexp(expected_exception, expected_regexp):

The version ending “p” was deprecated in Python 3.2 and will be removed in 3.12: https://docs.python.org/dev/whatsnew/3.12.html#removed .