bpo-36366: Return None on stopping unstarted patch object (GH-12472)

Return None after calling unittest.mock.patch.object.stop() regardless of whether the object was started. This makes the method idempotent.


https://bugs.python.org/issue36366

Lib/unittest/mock.py

@@ -1398,7 +1398,7 @@ class _patch(object):
     def __exit__(self, *exc_info):
         """Undo the patch."""
         if not _is_started(self):
-            raise RuntimeError('stop called on unstarted patcher')
+            return
 
         if self.is_local and self.temp_original is not DEFAULT:
             setattr(self.target, self.attribute, self.temp_original)