* requests: allow strings in Session.verify
Per the documentation[1] (and actual usage), the verify parameter can be a
string or a bool.
[1] http://docs.python-requests.org/en/master/user/advanced/#ssl-cert-verification
* requests: explicitly support AuthBase for Session.auth
According to the documentation, the auth parameter should be an AuthBase
subclass[1]. In practice, the actual requirement seems to just be a
Callable[[Request], Request], and AuthBase implements that with the __call__
method. However, mypy isn't currently smart enough to infer that __call__
implies Callable[2]. In the interim (and perhaps also to add clearer errors),
explicitly support AuthBase.
Additionally, this adds typing of AuthBase.__call__, to match the
Callable[[Request], Request] declaration used elsewhere.
[1] http://docs.python-requests.org/en/master/user/advanced/#custom-authentication
[2] https://github.com/python/mypy/issues/797
