[issue24991] Define instance mutability explicitly on type objects

[Nathaniel Smith]

Nathaniel Smith added the comment:

Adding Eric Snow to nosy because it seems like there may be some natural overlap between this and the per-subinterpreter GIL ideas he brought up on python-ideas back in June.

----------
nosy: +eric.snow

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue24991>
_______________________________________