update Certbot's mock usage

certbot/certbot/tests/util.py

@@ -6,12 +6,19 @@ import shutil
 import sys
 import tempfile
 import unittest
+import warnings
 
 from cryptography.hazmat.backends import default_backend
 from cryptography.hazmat.primitives import serialization
 import josepy as jose
 try:
     import mock
+    warnings.warn(
+        "The external mock module is being used for backwards compatibility "
+        "since it is available, however, future versions of Certbot's tests will "
+        "use unittest.mock. Be sure to update your code accordingly.",
+        PendingDeprecationWarning
+    )
 except ImportError: # pragma: no cover
     from unittest import mock # type: ignore
 import OpenSSL

certbot/setup.py

@@ -72,14 +72,6 @@ elif os.name == 'nt':
     # setuptools, pywin32 will not be specified as a dependency.
     install_requires.append(pywin32_req)
 
-if setuptools_known_environment_markers:
-    install_requires.append('mock ; python_version < "3.3"')
-elif 'bdist_wheel' in sys.argv[1:]:
-    raise RuntimeError('Error, you are trying to build certbot wheels using an old version '
-                       'of setuptools. Version 36.2+ of setuptools is required.')
-elif sys.version_info < (3,3):
-    install_requires.append('mock')
-
 dev_extras = [
     'coverage',
     'pytest',