Use ruff for linting

Use ruff instead of flake8. Aside from being substantially faster, ruff
includes a lot of flake's plug-ins built-in, some of which may be useful
to enable as a follow-up.

.ci/flake8.sh

-#!/bin/sh -e
-topdir="$(realpath "$(dirname "$0")/..")"
-cd "$topdir"
-
-flake8 $(find -name '*.py')
-
-echo "flake8 check passed"

.ci/ruff.sh

+#!/bin/sh -e
+# Description: lint all python scripts
+# https://postmarketos.org/pmb-ci
+
+if [ "$(id -u)" = 0 ]; then
+	set -x
+	apk -q add ruff
+	exec su "${TESTUSER:-build}" -c "sh -e $0"
+fi
+
+set -x
+
+# __init__.py with additional ignore:
+# F401: imported, but not used
+# shellcheck disable=SC2046
+ruff check --ignore "F401" $(find . -not -path '*/venv/*' -name '__init__.py')
+
+# Check all other files
+ruff check --exclude=__init__.py .
+
+# Check formatting
+ruff format --diff

.gitlab-ci.yml

@@ -24,15 +24,12 @@ mr-settings:
     - wget -q "https://gitlab.postmarketos.org/postmarketOS/ci-common/-/raw/master/check_mr_settings.py"
     - python3 ./check_mr_settings.py
 
-flake8:
+ruff:
   stage: check
   <<: *only-default
-  image: "alpine:latest"
-  before_script:
-  - "apk -q add py3-pip"
-  - "pip3 -q --disable-pip-version-check install flake8 --break-system-packages"
   script:
-  - ".ci/flake8.sh"
+    - "adduser -D build"
+    - ".ci/ruff.sh"
 
 upstream-compat-master:
   stage: check