Add gitlab-ci integration

.gitlab-ci.yml:
Add Gitlab CI integration, that runs tox's linter and coverage targets
in a lint stage.

.gitlab-ci.yml

+---
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+default:
+  image: "archlinux:latest"
+
+stages:
+  - lint
+
+.unit_test:
+  stage: lint
+  before_script:
+    - pacman --noconfirm -Syu --needed base-devel python-poetry python-tox
+
+linter:
+  extends: .unit_test
+  script:
+    - tox -e linter
+
+coverage:
+  extends: .unit_test
+  script:
+    - tox -e coverage