Gitlab ci runner setup

ok, I've attached a little runner (an old laptop of mine ;)), and written a gitlab-ci script that runs the tests and also checks for 'black' code-style, all seems to be passing right now (after another small fix): https://gitlab.aei.uni-hannover.de/GregAshton/PyFstat/pipelines

/cc @dkeitel

.gitlab-ci.yml

@@ -31,8 +31,9 @@ pytest:
     when: always
     expire_in: 24h
 
-.static:
+static:
   stage: Static Analysis
+  tags: [ pyFstat ]
   script:
     - source ${VENV_DIR}/bin/activate
     - black --check .