Fix signature of return_ and raise_

This is an oversight of patch 74c3c9b (Avoid `partial` when constructing
answers).  The catch all arguments we had because we used `partial` on
the functions.  Since we now don't there is no use for them.  They look
rather confusing, as if we the intention would be to eat all but the
first argument.

Hence, the cleanup.

mockito/invocation.py

@@ -405,13 +405,13 @@ def check_used(self):
                 "\nUnused stub: %s" % self)
 
 
-def return_(value, *a, **kw):
-    def answer(*a, **kw):
+def return_(value):
+    def answer(*args, **kwargs):
         return value
     return answer
 
-def raise_(exception, *a, **kw):
-    def answer(*a, **kw):
+def raise_(exception):
+    def answer(*args, **kwargs):
         raise exception
     return answer