This declares the method to take the union of Set and FrozenSet
rather than AbstractSet since that is where `difference()`
(used by `assertSetEqual()`) is defined.
The python2 unittest `assertIsInstance` and `assertNotIsInstance` stub
did not allow using a tuple of classes, but that behavior is [in the
documentation][1]. This commit copies the type stub for the isinstance
built-in to the stubs for `assertIsInstance` and `assertNotIsInstance`
I made this commit in response to @gvanrossum's request in
python/typeshed#802 (which fixed this issue for python 3) that I apply
the same fix to python 2.
[1]: https://docs.python.org/2.7/library/unittest.html#unittest.TestCase.assertNotIsInstance
