Needed tests for Hashable, Awaitable, Coroutine, AsyncIterable, AsyncIterator, Iterator, Generator, Sized, Container, Callable.
The patch adds some tests for __iadd__ and __eq__. I think needed tests for a number of other special methods. In particular should be tested a classes without __iadd__, but with __add__ but to None; with __radd__, but with __add__ set to None; without __add__, but with __radd__ set to None; with __eq__, but with __ne__ set to None, etc.
Added other comments on Rietveld.