Revert the last two mock PRs (#7903)

* Revert "Do not require mock in Python 3 in certbot module (#7895)"

This reverts commit 77871ba71c.

* Revert "Do not require mock in Python 3 in acme module (#7894)"

This reverts commit cd0acf5dcc.

certbot/tests/display/completer_test.py

@@ -7,10 +7,7 @@ import string
 import sys
 import unittest
 
-try:
-    import mock
-except ImportError: # pragma: no cover
-    from unittest import mock
+import mock
 from six.moves import reload_module  # pylint: disable=import-error
 
 from certbot.compat import filesystem  # pylint: disable=ungrouped-imports

certbot/tests/display/ops_test.py

@@ -4,10 +4,7 @@ import sys
 import unittest
 
 import josepy as jose
-try:
-    import mock
-except ImportError: # pragma: no cover
-    from unittest import mock
+import mock
 import zope.component
 
 from acme import messages

certbot/tests/display/util_test.py

@@ -4,10 +4,7 @@ import socket
 import tempfile
 import unittest
 
-try:
-    import mock
-except ImportError: # pragma: no cover
-    from unittest import mock
+import mock
 import six
 
 from certbot import errors