Lock the ruff version in tox.ini.

Previously, we'd install whatever is the latest ruff, but ruff can introduce new
warnings/errors which would then cause CI to break even if the same change worked
previously. This is undesirable as it can require significant changes to a PR just
to pass again.

tox.ini

@@ -35,7 +35,7 @@ commands =
 description =
   miscellaneous checks on the source code, including pyflakes
 deps =
-  ruff
+  ruff==0.4.2
 commands =
   python scripts/check_crlf.py pygments external
   python scripts/detect_missing_analyse_text.py --skip-no-aliases