Replace flake8 with ruff

[Ruff](https://beta.ruff.rs/) supports [over 500 lint rules](https://beta.ruff.rs/docs/rules) and can be used to replace [Flake8](https://pypi.org/project/flake8/) (plus dozens of plugins), [isort](https://pypi.org/project/isort/), [pydocstyle](https://pypi.org/project/pydocstyle/), [yesqa](https://github.com/asottile/yesqa), [eradicate](https://pypi.org/project/eradicate/), [pyupgrade](https://pypi.org/project/pyupgrade/), and [autoflake](https://pypi.org/project/autoflake/), all while executing (in Rust) tens or hundreds of times faster than any individual tool.

We use `ruff --format=gitlab` to rapidly provide intuitive feedback to contributors.

doc/ci/caching/index.md

@@ -331,8 +331,8 @@ before_script:
 test:
   script:
     - python setup.py test
-    - pip install flake8
-    - flake8 .
+    - pip install ruff
+    - ruff --format=gitlab .
 ```
 
 ### Cache Ruby dependencies