Set up ruff so that test files have at least some linting (#10399)

Alternative implementation for #7908.

In this PR:
- set up ruff in CI (add to `tox.ini`, mark dep in `certbot/setup.py`)
- add a `ruff.toml` that ignores particularly annoying errors. I think
line length isn't actually necessary to set with this workflow since
we're not checking it but putting it there for future usage.
- either fix or ignore the rest of the errors that come with the default
linting configuration. fixed errors are mostly unused variables. ignored
are usually where we're doing weird import things for a specific reason.

certbot/setup.py

@@ -71,6 +71,7 @@ test_extras = [
     'pytest',
     'pytest-cov',
     'pytest-xdist',
+    'ruff',
     'setuptools',
     'tox',
     'types-httplib2',