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] python/mypy#797

Author: dehnert

third_party/2and3/requests/auth.pyi

@@ -3,6 +3,7 @@
 from typing import Any
 from . import compat
 from . import cookies
+from . import models
 from . import utils
 from . import status_codes
 
@@ -15,7 +16,7 @@ CONTENT_TYPE_FORM_URLENCODED = ...  # type: Any
 CONTENT_TYPE_MULTI_PART = ...  # type: Any
 
 class AuthBase:
-    def __call__(self, r): ...
+    def __call__(self, r: models.Request) -> models.Request: ...
 
 class HTTPBasicAuth(AuthBase):
     username = ...  # type: Any

third_party/2and3/requests/sessions.pyi

@@ -16,6 +16,7 @@ from . import adapters
 from . import status_codes
 
 BaseAdapter = adapters.BaseAdapter
+AuthBase = auth.AuthBase
 OrderedDict = compat.OrderedDict
 cookiejar_from_dict = cookies.cookiejar_from_dict
 extract_cookies_to_jar = cookies.extract_cookies_to_jar
@@ -60,7 +61,7 @@ _Data = Union[None, bytes, MutableMapping[Text, Text], IO]
 class Session(SessionRedirectMixin):
     __attrs__ = ...  # type: Any
     headers = ...  # type: Optional[MutableMapping[Text, Text]]
-    auth = ...  # type: Union[None, Tuple[Text, Text], Callable[[Request], Request]]
+    auth = ...  # type: Union[None, Tuple[Text, Text], AuthBase, Callable[[Request], Request]]
     proxies = ...  # type: Optional[MutableMapping[Text, Text]]
     hooks = ...  # type: Optional[MutableMapping[Text, Callable[[Request], Any]]]
     params = ...  # type: Union[None, bytes, MutableMapping[Text, Text]]
@@ -82,7 +83,7 @@ class Session(SessionRedirectMixin):
                 headers: Optional[MutableMapping[Text, Text]] = ...,
                 cookies: Union[None, RequestsCookieJar, MutableMapping[Text, Text]] = ...,
                 files: Optional[MutableMapping[Text, IO]] = ...,
-                auth: Union[None, Tuple[Text, Text], Callable[[Request], Request]] = ...,
+                auth: Union[None, Tuple[Text, Text], AuthBase, Callable[[Request], Request]] = ...,
                 timeout: Union[None, float, Tuple[float, float]] = ...,
                 allow_redirects: Optional[bool] = ...,
                 proxies: Optional[MutableMapping[Text, Text]] = ...,