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As far as I can tell, if a new class does not add any new fields, and its base class doesn't use GC, there's no reason to enable GC for the new class.
This is useful for creating lightweight wrappers around classes implemented in C.

>>> class UserIntSlots(int):
... __slots__ = ()
... def __repr__(s): return '5'
... 
>>> (4).__class__ = UserIntSlots
>>> 2+2
5
>>> type(2+2)
<class '__main__.UserIntSlots'>
It looks weird.

Agreed, but this is not new. This works without my change:
>>> class Tuple(tuple):
... __slots__ = ()
... def __repr__(self): return 'Imma tuple!' 
... 
>>> ().__class__ = Tuple
>>> ()
Imma tuple!

I wasn't aware of that :-o

May be we should forbid assigning the __class__ attribute of basic builtin 
types (especially internable, such as int, str, bytes, tuple, bool, NoneType).

Actually, this is rather new -- new in 3.5. The check was relaxed in #22986: https://hg.python.org/cpython/rev/c0d25de5919e
Previously only heap types allowed re-assigning __class__.

New changeset a60b7945ef87 by Antoine Pitrou in branch 'default':
Issue #23726: Don't enable GC for user subclasses of non-GC types that don't add any new fields.
https://hg.python.org/cpython/rev/a60b7945ef87 

I've pushed the patch, thank you!

Thank you!
Benjamin, Nathaniel, any opinion if we should restrict reassigning __class__ for types like tuple, int and str, where some/many instances are cached?

Yes, it probably would be a good idea to disallow assigning __class__ for immutable types.

Agreed. There's a small problem with that, as far as I know. Nothing on type declares that it is immutable.