Make collections.abc.* generic

[emmatyping]

... and add tests for them.

I also removed __class_getitem__ from collections.UserDict/collections.UserList, since those inherit from collections.abc types (which is a nice test inheritance works for these types)!'

I didn't import types.GenericAlias in _collections_abc since this seems to be on the hot path for startup, and I figured that would be bad.

[gvanrossum]

Try running ./python.exe Lib/test/test_genericalias.py. There are three errors that occur without this too, but this one's new:

======================================================================
FAIL: test_collections_protocols_allowed (__main__.ProtocolTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/guido/pep585/Lib/test/test_typing.py", line 1381, in test_collections_protocols_allowed
    self.assertIsSubclass(B, Custom)
  File "/Users/guido/pep585/Lib/test/test_typing.py", line 40, in assertIsSubclass
    raise self.failureException(message)
AssertionError: <class '__main__.ProtocolTests.test_collections_protocols_allowed.<locals>.B'> is not a subclass of <class '__main__.ProtocolTests.test_collections_protocols_allowed.<locals>.Custom'>

[emmatyping]

The test failure was due to protocol runtime attribute compatibility checking. I disabled checking for __class_getitem__ because this would break existing code like in the test:

@runtime_checkable
class Custom(collections.abc.Iterable, Protocol):
        def close(self): ...

class B: # would now require an implementation of __class_getitem__
       def __iter__(self):
            return []
       def close(self):
            return 0

[gvanrossum]

You said that __class_getitem__ is inherited -- maybe you need fewer of them?

[emmatyping]

You said that class_getitem is inherited -- maybe you need fewer of them?

Yep! Done.

[gvanrossum]

Awesome! You left a few blank lines, in, but I'll take them.