Tests: tox and mock cleanup

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

tests/test_amazon_ses_backend.py

@@ -1,11 +1,11 @@
 import json
 from datetime import datetime
 from email.mime.application import MIMEApplication
+from unittest.mock import ANY, patch
 
 from django.core import mail
 from django.core.mail import BadHeaderError
 from django.test import SimpleTestCase, override_settings, tag
-from mock import ANY, patch
 
 from anymail.exceptions import AnymailAPIError, AnymailUnsupportedFeature
 from anymail.inbound import AnymailInboundMessage