add __init__ to SimpleNamespace (#2207)

```In [11]: SimpleNamespace(x=3)
Out[11]: namespace(x=3)
```

stdlib/3/types.pyi

@@ -79,6 +79,7 @@ class MappingProxyType(Mapping[_KT, _VT], Generic[_KT, _VT]):
     def __len__(self) -> int: ...
 
 class SimpleNamespace:
+    def __init__(self, **kwargs: Any) -> None: ...
     def __getattribute__(self, name: str) -> Any: ...
     def __setattr__(self, name: str, value: Any) -> None: ...
     def __delattr__(self, name: str) -> None: ...