TST: Check that system Python of base image is 3.9

.gitlab-ci.yml

@@ -129,6 +129,8 @@ unittest_py39:
   image: $CI_REGISTRY_IMAGE
   needs: [build-testenv]
   script:
+    # First verify that system Python actually is 3.9
+    - python -c "import sys; assert sys.version.startswith('3.9')"
     - python3 -c "import caosdb; print('CaosDB Version:', caosdb.__version__)"
     - tox