98c782b3 breaks Postorius

After updating to the latest mailmanclient, django-admin compress fails because postorius/templatetags/membership_helpers.py contains

from mailmanclient._client import MailingList

and there is no mailmanclient._client.

There are other references to mailmanclient._client but they are all in tests. They are in hyperkitty/tests/lib/test_posting.py, postorius/tests/mailman_api_tests/test_subscriptions.py and 3 in postorius/tests/mailman_api_tests/test_user.py