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Description
The whole _client.client indirection is pretty awkward and confuses me all the time. It might be time to get rid of the requests_oauthlib.OAuth2 _client by copying in it's functionality directly into the OAuth2Session.
That together with new and coming changes to oauthlibs OAuth 2 client (e.g. full prepare_x_request methods and state management) should make the OAuth2Session a bit clearer as well as leaner.
example of _client being an annoying indirection #153