Fix Windows CI job

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

.gitlab-ci.yml

@@ -100,6 +100,7 @@ minimal-windows:
     - source activate pystencils_dev
     - env
     - conda env list
+    - conda install `grep -Eo 'sympy[>=]+[0-9\.]+' setup.py | sed 's/>/=/g'`
     - 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