Mark a protocol class as a runtime protocol.
Such protocol can be used with isinstance() and issubclass(). Raise TypeError if applied to a non-protocol class. This allows a simple-minded structural check very similar to one trick ponies in collections.abc such as Iterable. For example:
@runtime_checkable class Closable(Protocol): def close(self): ... assert isinstance(open('/some/file'), Closable)
Warning: this will check only the presence of the required methods, not their type signatures!