Do not require mock in Python 3 in certbot-dns modules (#7900)

Part of #7886.

This PR conditionally installs `mock` in `certbot-dns-*/setup.py` based on setuptools version and python version, when possible. It then updates the tests to use `unittest.mock` when `mock` isn't available.

* Do not require mock in Python 3 in certbot-dns modules

* update changelog

* error when trying to build wheels with old setuptools

* add type: ignores

certbot-dns-sakuracloud/tests/dns_sakuracloud_test.py

@@ -2,7 +2,10 @@
 
 import unittest
 
-import mock
+try:
+    import mock
+except ImportError: # pragma: no cover
+    from unittest import mock # type: ignore
 from requests.exceptions import HTTPError
 
 from certbot.compat import os