Remove mock dependency (#8630)

Fixes https://github.com/certbot/certbot/issues/7913.

I only added the deprecation warning to `certbot.tests.util` because that's the only place where I think someone could be using the `mock` module through our API.

* remove external mock from acme

* update Certbot's mock usage

* remove mock dependency in plugins

* remove external mock from compatibility test

* add changelog entry

certbot-dns-rfc2136/setup.py

@@ -1,8 +1,6 @@
-from distutils.version import LooseVersion
 import os
 import sys
 
-from setuptools import __version__ as setuptools_version
 from setuptools import find_packages
 from setuptools import setup
 
@@ -27,15 +25,6 @@ elif 'bdist_wheel' in sys.argv[1:]:
 if os.environ.get('SNAP_BUILD'):
     install_requires.append('packaging')
 
-setuptools_known_environment_markers = (LooseVersion(setuptools_version) >= LooseVersion('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',