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-rfc2136/setup.py

@@ -1,5 +1,7 @@
+from distutils.version import StrictVersion
 import sys
 
+from setuptools import __version__ as setuptools_version
 from setuptools import find_packages
 from setuptools import setup
 from setuptools.command.test import test as TestCommand
@@ -12,11 +14,19 @@ install_requires = [
     'acme>=0.29.0',
     'certbot>=1.1.0',
     'dnspython',
-    'mock',
     'setuptools',
     'zope.interface',
 ]
 
+setuptools_known_environment_markers = (StrictVersion(setuptools_version) >= StrictVersion('36.2'))
+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')
+
 docs_extras = [
     'Sphinx>=1.0',  # autodoc_member_order = 'bysource', autodoc_default_flags
     'sphinx_rtd_theme',

certbot-dns-rfc2136/tests/dns_rfc2136_test.py

@@ -5,7 +5,10 @@ import unittest
 import dns.flags
 import dns.rcode
 import dns.tsig
-import mock
+try:
+    import mock
+except ImportError: # pragma: no cover
+    from unittest import mock # type: ignore
 
 from certbot import errors
 from certbot.compat import os