Change an optional scopes parameter to be required

The previous default None value would cause an exception in our code decorate_scope().
The fix is to change the previously optional scopes parameter to required.
This kind of api surface change would usually be a breaking change, but in this case it is not,
because the previous default value would cause exception so it was in fact required.
FWIW, you can still explicitly use empty list [] as scope,
and the response would contain id_token and refresh_token, but no access_token.

Author: rayluo

msal/application.py

@@ -512,7 +512,7 @@ def acquire_token_by_device_flow(self, flow, **kwargs):
                 **kwargs)
 
     def acquire_token_by_username_password(
-            self, username, password, scopes=None, **kwargs):
+            self, username, password, scopes, **kwargs):
         """Gets a token for a given resource via user credentails.
 
         See this page for constraints of Username Password Flow.