Erlend: would you mind to document the new Py_TPFLAGS_IMMUTABLETYPE flag in the C API section of What's New in Python 3.10?
https://docs.python.org/dev/whatsnew/3.10.html#c-api-changes
In "New Features", something like:
* Add a new :c:data:`Py_TPFLAGS_IMMUTABLETYPE` type flag to (...).
(Contributed by xxx in :issue:`43908`.)
And in the "Porting to Python 3.10", I don't know if we should suggest reviewing metatypes implemented in C which overrides tp_setattro, to take the new flag in account.
Maybe explain that Py_TPFLAGS_IMMUTABLETYPE should now be checked to decide if a type is mutable or not, rather than relying on Py_TPFLAGS_HEAPTYPE.