Tests: tox and mock cleanup

* Remove mock dependency
  (mock is now part of standard unittest package)
* Cleanup tox dependency installation logic

tests/test_message.py

@@ -1,6 +1,7 @@
+from unittest.mock import patch
+
 from django.core.mail import EmailMultiAlternatives
 from django.test import SimpleTestCase
-from mock import patch
 
 from anymail.message import AnymailRecipientStatus, AnymailStatus, attach_inline_image