Asserts that the message in a triggered warning matches a regexp.
Basic functioning is similar to assertWarns() with the addition
that only warnings whose messages also match the regular expression
are considered successful matches.
Args:
expected_warning: Warning class expected to be triggered.
expected_regex: Regex (re.Pattern object or string) expected
to be found in error message.
args: Function to be called and extra positional args.
kwargs: Extra kwargs.
msg: Optional message used in case of failure. Can only be used
when assertWarnsRegex is used as a context manager.