fix CI (try 9)

.gitlab-ci.yml

@@ -23,5 +23,5 @@ test:
   script:
     - make -C Source tests
     - cd Source/Tests
-    - python run_tests.py
+    - python3 run_tests.py
   image: dpsim-dev

Source/Tests/run_tests.py

-#!/usr/bin/python
+#!/usr/bin/python3
 #
 # Script for automatically running the tests and comparing the results against
 # given CSV files (from some other simulator).