Do not require mock in Python 3 in acme module (#7910)

Part of #7886.

This PR conditionally installs mock in `acme/setup.py` based on setuptools version and python version, when possible. It then updates `acme` tests to use `unittest.mock` when `mock` isn't available.

Now with `type: ignore` as appropriate. Once the "future steps" of #7886 are finished, and mypy is on Python 3, the `pragma no cover`s and `type ignore`s will be gone.

* Conditionally install mock in acme

* error out on newer python and older setuptools

* error when trying to build wheels with old setuptools

* use unittest.mock when third-party mock isn't available in acme, with no cover and type ignore

acme/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
@@ -15,7 +17,6 @@ install_requires = [
     # 1.1.0+ is required to avoid the warnings described at
     # https://github.com/certbot/josepy/issues/13.
     'josepy>=1.1.0',
-    'mock',
     # Connection.set_tlsext_host_name (>=0.13)
     'PyOpenSSL>=0.13.1',
     'pyrfc3339',
@@ -26,6 +27,15 @@ install_requires = [
     'six>=1.9.0',  # needed for python_2_unicode_compatible
 ]
 
+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')
+
 dev_extras = [
     'pytest',
     'pytest-xdist',