Updated inverse_of_unit in multi_polynomial_element

Changed inverse_of_unit in multi_polynomial_element so
that it no longer changes the base ring.

src/sage/rings/polynomial/multi_polynomial_element.py

@@ -926,7 +926,7 @@ def inverse_of_unit(self):
         if self.is_unit():
             if len(k) != 1:
                 raise NotImplementedError
-            return ~d[k[0]]
+            return d[k[0]].inverse_of_unit()
         raise ArithmeticError("is not a unit")
 
     def is_homogeneous(self):