ISTM that this would encourage silently inefficient coding patterns like:

   url.endswith({'.html', '.txt', '.php'})
   # The set object gets rebuilt on every call
   # and a new set iterator object gets built on every call.
   # Looping over the contents uses the slower iterator protocol
   # rather than the existing superfast PyTuple_GET_SIZE() and
   # PyTuple_GET_ITEM() macros which are entirely in-lined.

Do we have any known use cases or user requests where the existing API doesn't suffice?