Merge pull request #1398 from burnash/fix/oauth-re-authentication

check oauth creds type using `isinstance`

gspread/auth.py

@@ -198,7 +198,7 @@ def oauth(
     authorized_user_filename = Path(authorized_user_filename)
     creds = load_credentials(filename=authorized_user_filename)
 
-    if not type(creds) is Credentials:
+    if not isinstance(creds, Credentials):
         with open(credentials_filename) as json_file:
             client_config = json.load(json_file)
         creds = flow(client_config=client_config, scopes=scopes)