api: fix non-integer Mul args

Author: mloubout

devito/finite_differences/differentiable.py

@@ -552,10 +552,8 @@ def __new__(cls, *args, **kwargs):
         nums, others = split(args, lambda e: isinstance(e, (int, float,
                                                             sympy.Number, np.number)))
         scalar = sympy.Mul(*nums)
-        try:
+        if int(scalar) - scalar == 0:
             scalar = sympy.Integer(scalar)
-        except TypeError:
-            pass
 
         # a*0 -> 0
         if scalar == 0: