Fix patch.object to return a _patch context manager.

This should fix #914

Author: Roy Williams

stdlib/3/unittest/mock.pyi

@@ -92,7 +92,13 @@ if sys.version_info >= (3, 3):
         def start(self) -> Any: ...
         def stop(self) -> Any: ...
 
-    def patch(target: Any, new: Any =..., spec: Any = None, create: Any = False, spec_set: Any = None, autospec: Any = None, new_callable: Any = None, **kwargs: Any) -> Any: ...
+    class _patcher:
+        def __call__(self, target: Any, new: Any =..., spec: Any = None, create: Any = False, spec_set: Any = None, autospec: Any = None, new_callable: Any = None, **kwargs: Any) -> Any: ...
+        def object(self, target: Any, attribute: str, new: Any =..., spec: Any = None, create: Any = False, spec_set: Any = None, autospec: Any = None, new_callable: Any = None, **kwargs: Any) -> _patch: ...
+
+    patch = ...  # type: _patcher
+
+    patch = ...  # type: _patcher
 
     class _patch_dict:
         in_dict = ...  # type: Any