Bugfix: Pystencils printed `const` twice on assignments to const numpy.dtypes

Pystencils will print two const when printing an assignment to a const numpy.dtype.
With the new type interference, this case can now occur more often
8 jobs for bugfix-avoid-east-and-west-const in 4 minutes and 9 seconds (queued for 14 seconds)
latest
Name Stage Failure
failed
minimal-sympy-master Test
    visit_node(a)
File "/builds/seitz/pystencils/pystencils/transformations.py", line 526, in visit_node
sub_ast.lhs = visit_sympy_expr(sub_ast.lhs, enclosing_block, sub_ast)
File "/builds/seitz/pystencils/pystencils/transformations.py", line 504, in visit_sympy_expr
field_access.offsets, field_access.index)
File "/builds/seitz/pystencils/pystencils/astnodes.py", line 591, in __new__
assert isinstance(base.label, TypedSymbol)
AssertionError
ERROR: Job failed: exit code 1