Support urllib3 >= 2.0.0

The urllib3.request.RequestMethods has been moved to
urllib3._request_methods.RequestMethods.

This patch changes the usage to continue working with the latest
release, but now it's a "private" class in a "private" module, so maybe
it's worth to start to think about updating this code to use the public
API.

https://urllib3.readthedocs.io/en/stable/changelog.html#removed

Author: danigm

google/auth/transport/urllib3.py

@@ -95,8 +95,8 @@ class Request(transport.Request):
         credentials.refresh(request)
 
     Args:
-        http (urllib3.request.RequestMethods): An instance of any urllib3
-            class that implements :class:`~urllib3.request.RequestMethods`,
+        http (urllib3._request_methods.RequestMethods): An instance of any urllib3
+            class that implements :class:`~urllib3._request_methods.RequestMethods`,
             usually :class:`urllib3.PoolManager`.
 
     .. automethod:: __call__
@@ -184,7 +184,7 @@ def _make_mutual_tls_http(cert, key):
     return http
 
 
-class AuthorizedHttp(urllib3.request.RequestMethods):
+class AuthorizedHttp(urllib3._request_methods.RequestMethods):
     """A urllib3 HTTP class with credentials.
 
     This class is used to perform requests to API endpoints that require
@@ -197,8 +197,9 @@ class AuthorizedHttp(urllib3.request.RequestMethods):
         response = authed_http.request(
             'GET', 'https://www.googleapis.com/storage/v1/b')
 
-    This class implements :class:`urllib3.request.RequestMethods` and can be
-    used just like any other :class:`urllib3.PoolManager`.
+    This class implements
+    :class:`urllib3._request_methods.RequestMethods` and can be used
+    just like any other :class:`urllib3.PoolManager`.
 
     The underlying :meth:`urlopen` implementation handles adding the
     credentials' headers to the request and refreshing credentials as needed.

setup.py

@@ -29,7 +29,7 @@
     # install enum34 to support 2.7. enum34 only works up to python version 3.3.
     'enum34>=1.1.10; python_version < "3.4"',
     "six>=1.9.0",
-    "urllib3<2.0",
+    "urllib3>=2.0",
 )
 
 extras = {