Add extra assertions related to sympy issue [#17652](https://github.com/sympy/sympy/issues/17652)

8 jobs for extra-asserts-sympy-issue in 3 minutes and 8 seconds (queued for 2 seconds)
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Name Stage Failure
failed
minimal-sympy-master Test
    visit_node(a)
File "/builds/seitz/pystencils/pystencils/transformations.py", line 584, in visit_node
sub_ast)
File "/builds/seitz/pystencils/pystencils/transformations.py", line 558, in visit_sympy_expr
field_access.index)
File "/builds/seitz/pystencils/pystencils/astnodes.py", line 574, in __new__
assert isinstance(base.label, TypedSymbol)
AssertionError
ERROR: Job failed: exit code 1