Fix compatibility with Python 3.12 and newer.
Replace the deprecated and removed unittest.assertEquals() alias
with assertIsNone() to explicitly verify that the refresh token
is absent.
--- ../tests/contrib/test_devshell.py	2026-10-16 17:20:25.971174167 +0300
+++ ../tests/contrib/test_devshell.py	2026-10-16 17:20:46.500469592 +0300
@@ -205,7 +205,7 @@
     def test_no_refresh_token(self):
         with _AuthReferenceServer():
             creds = devshell.DevshellCredentials()
-            self.assertEquals(None, creds.refresh_token)
+            self.assertIsNone(creds.refresh_token)
 
     @mock.patch('oauth2client.client._UTCNOW')
     def test_reads_credentials(self, utcnow):