Bump minimum SymPy version and add Python 3.9 to CI

Same as pystencils!208 (merged)

.gitlab-ci.yml

@@ -69,6 +69,8 @@ test-longrun:
       - $ENABLE_NIGHTLY_BUILDS
   image: i10git.cs.fau.de:5005/pycodegen/pycodegen/full
   script:
+    - env 
+    - pip list
     - export NUM_CORES=$(nproc --all)
     - mkdir -p ~/.config/matplotlib
     - echo "backend:template" > ~/.config/matplotlib/matplotlibrc