:func:`~unittest.TestCase.assertRaises`,
:func:`~unittest.TestCase.assertRaisesRegex`,
:func:`~unittest.TestCase.assertWarns` and
:func:`~unittest.TestCase.assertWarnsRegex` no longer success if the passed
callable is None. They no longer ignore unknown keyword arguments in the
context manager mode. A DeprecationWarning was raised in these cases
since Python 3.5.
