Subclassed Symbols that define `__getitem__` break `DenseMatrix`

[bocklund]

Here's a self-contained test

from symengine import Symbol, DenseMatrix
class MySymbol(Symbol):
    def __getitem__(self, x):
        pass
x = Symbol("x")
y = MySymbol("y")
DenseMatrix([x])  # works fine
DenseMatrix([y])  # SIGSEGV

In PyCalphad (https://github.com/pycalphad/pycalphad/issues/547), we have something where __getitem__ returns, effectively, a new MySymbol, and think ends up in some loop where MySymbol keeps getting created and eats all the system memory.