Tests: tox and mock cleanup

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

tests/test_mailjet_inbound.py

@@ -1,8 +1,8 @@
 import json
 from base64 import b64encode
+from unittest.mock import ANY
 
 from django.test import tag
-from mock import ANY
 
 from anymail.inbound import AnymailInboundMessage
 from anymail.signals import AnymailInboundEvent