TST: Add python-version check to python 3.9 unittest

.gitlab-ci.yml

@@ -73,6 +73,8 @@ unittest_py3.9:
   stage: test
   needs: [ ]
   script:
+    # verify that this actually is Python 3.9
+    - python3 -c "import sys; assert sys.version.startswith('3.9')"
     - touch ~/.pycaosdb.ini
     - make unittest