chore(v1): Pin rsa to 4.5 for Python 2.7 compatibility (#1205)

As python 2.7 is no longer supported in master, this fix is only intended for the [v1](https://github.com/googleapis/google-api-python-client/tree/v1) branch where we still have support for python 2.7.

Fixes #1204 🦕

setup.py

diff --git a/setup.py b/setup.py
index bf0a8f4..8086749 100644
--- a/setup.py
+++ b/setup.py
@@ -39,6 +39,8 @@
     "google-auth>=1.16.0",
     "google-auth-httplib2>=0.0.3",
     "google-api-core>=1.21.0,<2dev",
+    # rsa version 4.5 is the last version that is compatible with Python 2.7
+    "rsa==4.5;python_version<'3'",
     "six>=1.13.0,<2dev",
     "uritemplate>=3.0.0,<4dev",
 ]

tests/test__auth.py

diff --git a/tests/test__auth.py b/tests/test__auth.py
index 9c4ea65..5923e8d 100644
--- a/tests/test__auth.py
+++ b/tests/test__auth.py
@@ -18,6 +18,7 @@
 import google_auth_httplib2
 import httplib2
 import oauth2client.client
+import pkg_resources
 import unittest2 as unittest
 
 from googleapiclient import _auth
@@ -82,6 +83,8 @@
         ):
             pass
 
+        google_auth_version = pkg_resources.get_distribution("google-auth").parsed_version
+
         credentials = mock.Mock(spec=CredentialsWithScopes)
         credentials.requires_scopes = True
 
@@ -89,7 +92,12 @@
 
         self.assertNotEqual(credentials, returned)
         self.assertEqual(returned, credentials.with_scopes.return_value)
-        credentials.with_scopes.assert_called_once_with(mock.sentinel.scopes)
+
+        # The `default_scopes` argument was added in google-auth==1.25.0
+        if google_auth_version >= pkg_resources.parse_version("1.25.0"):
+            credentials.with_scopes.assert_called_once_with(mock.sentinel.scopes, default_scopes=None)
+        else:
+            credentials.with_scopes.assert_called_once_with(mock.sentinel.scopes)
 
     def test_authorized_http(self):
         credentials = mock.Mock(spec=google.auth.credentials.Credentials)