MAINT: fix assert_raises_regex when used as a context manager

[shoyer]

Formerly, I got DeprecationWarnings about callable being None when I tested
on Python 3.

This should fix the currently failing tests in #8054.

[charris]

Hmm, checking shows that unittest has

    def assertRaisesRegex(self, expected_exception, expected_regex,
                          *args, **kwargs):

e.g., no mention of callable, although the official documentation includes it as a purely positional argument. I wonder if we should just omit the =None here in the documentation and mention that when used as a context manager only the first two arguments are needed. Or maybe go all long winded and just keep the first two arguments and document the parameters with callable_obj documented as being omitted when used as context manager. The Python 3.5 documentation has

            expected_exception: Exception class expected to be raised.
            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 assertRaisesRegex is used as a context manager.

with callable omitted from the signature. Unfortunately, msg is not available in 2.7.

[shoyer]

I cleaned up the docstring, and added the two function signatures from assertRaisesRegex:
https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertRaisesRegex

[charris]

This looks good, but the function documentation and signature can perhaps be improved.

[charris]

Thanks @shoyer .