Remove external mock dependency (#9331)

* Remove external mock dependency

This also removes the "external-mock" test environment

* remove superfluous ignores

* remove mock warning ignore from pytest.ini

* drop deps on mock in oldest, drop dep on types-mock

Co-authored-by: Alex Zorin <alex@zorin.id.au>

certbot-dns-cloudflare/tests/dns_cloudflare_test.py

@@ -1,12 +1,9 @@
 """Tests for certbot_dns_cloudflare._internal.dns_cloudflare."""
 
 import unittest
+from unittest import mock
 
 import CloudFlare
-try:
-    import mock
-except ImportError: # pragma: no cover
-    from unittest import mock # type: ignore
 
 from certbot import errors
 from certbot.compat import os