python: make typevar TAsyncClient/TSyncClient covariant
python: make typevar TAsyncClient/TSyncClient covariant

Summary: And introduce `TClient`, which is now the base class of all generated `Client`s.

Reviewed By: mvismonte, MaybebabyDuh

Differential Revision: D35731525

fbshipit-source-id: 3ed6632c9a5ac276af2886d5ae90f53ed257f7a5

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