_abc_registry errors with py3.8

[awais786]

With python3.8 tests are failing with this error.

    from typing import Any
  File "/.tox/django22/lib/python3.8/site-packages/typing.py", line 1357, in <module>
    class Callable(extra=collections_abc.Callable, metaclass=CallableMeta):
  File "/.tox/django22/lib/python3.8/site-packages/typing.py", line 1005, in __new__
    self._abc_registry = extra._abc_registry
AttributeError: type object 'Callable' has no attribute '_abc_registry'

python 3.7 is the first release where typing is not released as "provisional".

Please check the for detailed discussion.