Fix __init__ signature for unittest.mock.MagicMock. (#1388)

Author: carljm

stdlib/3/unittest/mock.pyi

@@ -118,7 +118,7 @@ if sys.version_info >= (3, 3):
         def mock_add_spec(self, spec: Any, spec_set: Any = False) -> Any: ...
 
     class MagicMock(MagicMixin, Mock):
-        def __init__(self) -> None: ...
+        def __init__(self, spec: Any = ..., spec_set: Any = ..., side_effect: Any = ..., return_value: Any = ..., wraps: Any = ..., name: Any = ..., **kwargs: Any) -> None: ...
         def mock_add_spec(self, spec: Any, spec_set: Any = False) -> Any: ...
 
     class MagicProxy: