CI: Check formatting with Ruff as well (MR 35)

If the list of files is omitted, Ruff will check all files recursively.

Use --diff to get a diff of what's not complicit instead of applying
the changes directly.

.ci/ruff.sh

@@ -12,3 +12,4 @@ set -x
 
 # shellcheck disable=SC2046
 ruff check $(find .ci -name '*.py')
+ruff format --diff