[Fix] Update for Docker Images

Due to an update of the docker images minor changes are required for the CI

.gitlab-ci.yml

@@ -101,13 +101,13 @@ minimal-windows:
     - py.test -v -m "not (notebook or longrun)"
 
 ubuntu:
-  stage: test
+  stage: pretest
   except:
     variables:
       - $ENABLE_NIGHTLY_BUILDS
   image: i10git.cs.fau.de:5005/pycodegen/pycodegen/ubuntu
   before_script:
-    - apt-get -y remove python3-sympy
+    # - apt-get -y remove python3-sympy
     - ln -s /usr/include/locale.h /usr/include/xlocale.h
     - pip3 install sympy==1.6
     # - pip3 install `grep -Eo 'sympy[>=]+[0-9\.]+' setup.py | sed 's/>/=/g'`