sympy_parser: don't apply function exponentation on Symbols

Fixes a bug where defining a symbol and then attempting to exponentiate it caused the implicit transformations to fail.

This used to cause an error, as the parser believed that `x` was now a function:

    transformations = standard_transformations + (convert_xor, function_exponentiation)
    parse_expr('x**2', local_dict={ 'x': sympy.Symbol('x') },
                      transformations=transformations)