CI: add flake8 check

Let's get meta: make sure that the common-ci scripts are linted with
flake8 as well.

.ci/flake8.sh

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

.gitlab-ci.yml

+---
+flake8:
+  image: "alpine:latest"
+  before_script:
+  - "apk -q add py3-pip"
+  - "pip3 -q --disable-pip-version-check install flake8"
+  script:
+  - ".ci/flake8.sh"