cleanup: polyfill assertRaisesRegex so we can avoid assertRaisesRegexp The latter is deprecated on Python 3.7 and causes our tests to fail due to the warning. Differential Revision: https://phab.mercurial-scm.org/D3375

To run the tests, do:

cd tests/
python run-tests.py

See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.