Add typing support to torch._six (#42232)
Summary:
Also add __prepare__ method metaclass created by `with_metaclass` to conform with PEP 3115
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42232
Reviewed By: ezyang
Differential Revision: D22816936
Pulled By: malfet
fbshipit-source-id: a47d054b2f061985846d0db6b407f4e5df97b0d4
