ci: replace Flake8 with Ruff (MR 4741)

See similar patch in pmbootstrap for rationale:
27618d5ffd

W504 ignore is removed as the rule does not exist in Ruff. But, since it
was ignored anyway, this does not matter.

.ci/ruff.sh

@@ -4,11 +4,11 @@
 
 if [ "$(id -u)" = 0 ]; then
 	set -x
-	apk -q add py3-flake8
+	apk -q add ruff
 	exec su "${TESTUSER:-build}" -c "sh -e $0"
 fi
 
 set -x
 
 # shellcheck disable=SC2046
-flake8 --ignore E501,F401,E722,W504,W605 $(find .ci -name '*.py')
+ruff --ignore E501,F401,E722,W605 $(find .ci -name '*.py')

.gitlab-ci.yml

@@ -36,7 +36,7 @@ wiki:
     - .ci/wiki.sh
 
 # testcases linting
-flake8:
+ruff:
   stage: lint
   <<: *only-default
   only:
@@ -46,10 +46,11 @@ flake8:
       - .ci/*
       - .ci/*/*
   before_script:
-    - apk -q add py-flake8
+    - "echo 'https://dl-cdn.alpinelinux.org/alpine/edge/testing' >> /etc/apk/repositories"
+    - apk -q add ruff
   script:
     - .ci/lib/gitlab_prepare_ci.sh
-    - .ci/flake8.sh
+    - .ci/ruff.sh
 
 # shellcheck and various grep checks
 shellcheck-grep: