ci: Move shellcheck/black/flake8 checks to a new CI job which can fail

We were regressing on Python style too often. Since Python code style is
a lot easier to enforce than C code style, split it (and the shellcheck
checks) out from `style-check-diff` into a new CI job which is allowed
to fail the pipeline.

Only trigger it when .sh or .py files have changed, which should reduce
resource consumption.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

.gitlab-ci.yml

@@ -55,10 +55,21 @@ style-check-diff:
   allow_failure: true
   script:
     - .gitlab-ci/run-style-check-diff.sh
+    - .gitlab-ci/run-check-todos.sh
+
+sh-and-py-check:
+  extends: .only-default
+  image: $DEBIAN_IMAGE
+  stage: style-check
+  allow_failure: false
+  script:
     - .gitlab-ci/run-shellcheck.sh
     - .gitlab-ci/run-black.sh
     - .gitlab-ci/run-flake8.sh
-    - .gitlab-ci/run-check-todos.sh
+  only:
+    changes:
+      - "**/*.py"
+      - "**/*.sh"
 
 fedora-x86_64:
   extends: