[Python-Dev] Restrict the type of __slots__

Currently __slots__ can be either string or an iterable of strings. 1. If it is a string, it is a name of a single slot. Third-party code which iterates __slots__ will be confused. 2. If it is an iterable, it should emit names of slots. Note that non-reiterable iterators are accepted too, but it causes weird bugs if __slots__ is iterated more than once. For example it breaks default pickling and copying. I propose to restrict the type of __slots__. Require it always been a tuple of strings. Most __slots__ in real code are tuples. It is rarely we need only single slot and set __slots__ as a string. It will break some code (there are 2 occurrences in the stdlib an 1 in scripts), but that code can be easily fixed.