Fix Expr.__new__(Symbol) error

Author: isuruf

symengine/lib/symengine_wrapper.pyx

@@ -63,7 +63,7 @@ cdef object c2py(rcp_const_basic o):
     elif (symengine.is_a_Symbol(deref(o))):
         if (symengine.is_a_PySymbol(deref(o))):
             return <object>(deref(symengine.rcp_static_cast_PySymbol(o)).get_py_object())
-        r = Expr.__new__(Symbol)
+        r = Symbol.__new__(Symbol)
     elif (symengine.is_a_Constant(deref(o))):
         r = S.Pi
         if (symengine.eq(deref(o), deref(r.thisptr))):