Fix Windows CI job

Failing builds (https://i10git.cs.fau.de/pycodegen/lbmpy/-/jobs/530612) since !57 (merged).

.gitlab-ci.yml

@@ -102,7 +102,7 @@ minimal-windows:
     - conda env list
     - grep sympy setup.py
     - grep -Eo 'sympy[>=]+[0-9\.]+' setup.py | sed 's/>/=/g'
-    - conda install sympy==1.2
+    - conda update sympy==1.2
     - pip install git+https://gitlab-ci-token:${CI_JOB_TOKEN}@i10git.cs.fau.de/pycodegen/pystencils.git@master#egg=pystencils
     - python -c "import numpy"
     - python setup.py quicktest