Use typing-extensions to ensure certbot dev environment is compatible with Python 3.6/3.7 (#8776)

Fixes #8773

I took option 2 from the issue mentionned above (importing `typing-extensions` on dev dependencies) to avoid modifying certbot runtime requirements given that what needs to be added is useful for mypy only.

I did not change the Python version used to execute the linting and mypy on the standard tests, given that the tox `docker_dev` target already checks if the development environment is working for Python < 3.8.

certbot/certbot/plugins/dns_test_common.py

@@ -12,7 +12,7 @@ from certbot.tests import acme_util
 from certbot.tests import util as test_util
 
 if typing.TYPE_CHECKING:
-    from typing import Protocol
+    from typing_extensions import Protocol
 else:
     Protocol = object  # type: ignore
 

certbot/certbot/plugins/dns_test_common_lexicon.py

@@ -18,7 +18,7 @@ try:
 except ImportError:  # pragma: no cover
     from unittest import mock  # type: ignore
 if typing.TYPE_CHECKING:
-    from typing import Protocol
+    from typing_extensions import Protocol
 else:
     Protocol = object  # type: ignore
 

certbot/setup.py

@@ -77,6 +77,9 @@ dev_extras = [
     'pytest',
     'pytest-cov',
     'pytest-xdist',
+    # typing-extensions is required to import typing.Protocol and make the mypy checks
+    # pass (along with pylint about non-existent objects) on Python 3.6 & 3.7
+    'typing-extensions',
     'tox',
     'twine',
     'wheel',