Require sympy>=1.12.1 in tests

So far, sympy 1.11 was used due to pysb, but this is incompatible with numpy>=2.0

Author: dweindl

tox.ini

@@ -14,6 +14,7 @@ description =
 [testenv:unit]
 extras = tests,reports,combine,vis
 commands =
+  python -m pip install sympy>=1.12.1
   python -m pytest --cov=petab --cov-report=xml --cov-append \
     tests
 description =