ci: switch from flake8 to pyproject-flake8

.gitlab-ci.yml

@@ -42,8 +42,8 @@ Check style:
   variables:
     STYLE_PATCH: ${CI_PROJECT_DIR}/style.patch
   before_script:
-    - apk add black clang-extra-tools git py3-flake8 py3-isort py3-pip python3
-    - python -m pip install --root-user-action=ignore --break-system-packages cmake-format fprettify
+    - apk add black clang-extra-tools git py3-isort py3-pip python3
+    - python -m pip install --root-user-action=ignore --break-system-packages cmake-format fprettify pyproject-flake8
   script:
     # Exclude the automatically generated mtime_iso8601.c:
     - c_cxx_files=$(find "${CI_PROJECT_DIR}" -name 'mtime_iso8601.c' -prune -o \( -name '*.h' -o -name '*.c' \) -print)