code: def clean_username(self, username):
          
          username_case = settings.CAS_FORCE_CHANGE_USERNAME_CASE
          if username_case == 'lower':
              username = username.lower()
          elif username_case == 'upper':
              username = username.upper()
          elif username_case is not None:
              raise ImproperlyConfigured(
                  "Invalid value for the CAS_FORCE_CHANGE_USERNAME_CASE setting. "
                  "Valid values are `'lower'`, `'upper'`, and `None`.")
          return username
docs: Performs any cleaning on the "username" prior to using it to get or
          create the user object.  Returns the cleaned username.
  
          By default, changes the username case according to
          `settings.CAS_FORCE_CHANGE_USERNAME_CASE`.