Upgrade SymPy Version & Extend CI

.gitlab-ci.yml

conftest.py

pyproject.toml

src/lbmpy/phasefield/post_processing.py

src/lbmpy/sparse/mapping.py

tests/cumulantmethod/test_flow_around_sphere.py

tests/test_fluctuating_lb.py

tests/test_sparse_lbm.ipynb

tests/test_stokes_setup.ipynb