Remove unnecessary unittest (#9596)

Now that we're using pytest more aggressively, I think we should start transitioning our tests to that style rather than continuing to use unittest. This PR removes some unnecessary uses of unittest I found.

I kept the test classes (while removing the inheritance from unittest.TestCase) where I felt like it added structure or logical grouping of tests.

I verified that pytest still finds all the tests in both this branch and master by running commands like:
```
pytest $(git diff --name-only master | grep -v windows_installer_integration_tests)
```

certbot-ci/windows_installer_integration_tests/test_main.py

@@ -4,12 +4,11 @@ import re
 import subprocess
 import time
 from typing import Any
-import unittest
 
 import pytest
 
 
-@unittest.skipIf(os.name != 'nt', reason='Windows installer tests must be run on Windows.')
+@pytest.mark.skipif(os.name != 'nt', reason='Windows installer tests must be run on Windows.')
 def test_it(request: pytest.FixtureRequest) -> None:
     try:
         subprocess.check_call(['certbot', '--version'])