add CI configuration

.flake8

+[flake8]
+max-line-length = 120

.gitlab-ci.yml

+---
+
+image: registry.git.fsmpi.rwth-aachen.de/infra/ci-containers/fsmpi-python:buster
+
+before_script:
+  - export LANG=en_US.UTF-8
+
+stages:
+  - test
+
+test:
+  stage: test
+  script:
+    - "python3 -m py_compile *.py"
+    - flake8 .