[python] fix wait function for custom List
    
    The `wait` function is supposed to work with on `waitable` instances.
    It should work on a `Munch` returned by other API method (i.e. it has
    a `__proxy__` property), or `list` of such munches.
    
    It should also work on a `helpers.List` of `Munch` objects returned
    by some API method. In such case, we set the `__proxy__` property
    to the `List` not munches themselves.