Updating to Mailman Core 3.1 API can cause issues due to cache

It was reported shortly after Django-mailman3 1.3.3 was released that some users were getting 404 errors on Mailman options page.

After investigation, it was found that because user_ids are cached by django-mailman3, when updating from 3.0 to 3.1 API, the old user_ids when used with 3.1 API would return 404 since the format of the user_id changed from integer to uuid.