More precise typing for MainClass.get_user() (#1575)

Use typing.overload for MainClass.get_user() since it can return two
disparate types. Correct the docstring for it, as well as locking down
Sphinx to <3 due to built docs issue and drive-by link correction.

Fixes #1550

github/MainClass.py

@@ -257,7 +257,7 @@ class Github(object):
         """
         :calls: `GET /users/:user <http://developer.github.com/v3/users>`_ or `GET /user <http://developer.github.com/v3/users>`_
         :param login: string
-        :rtype: :class:`github.NamedUser.NamedUser`
+        :rtype: :class:`github.NamedUser.NamedUser` or :class:`github.AuthenticatedUser.AuthenticatedUser`
         """
         assert login is github.GithubObject.NotSet or isinstance(login, str), login
         if login is github.GithubObject.NotSet: