Fix signature of return_ and raise_

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