[issue39573] Make PyObject an opaque structure in the limited C API

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

> it can prepare CPython to experiment tagged pointers

In September 2018, Neil Schemenauer did an experiment:

* https://mail.python.org/archives/list/capi-sig@python.org/thread/EGAY55ZWMF2WSEMP7VAZSFZCZ4VARU7L/
* https://github.com/nascheme/cpython/commits/tagged_int

More recent discussion on the capi-sig list:

https://mail.python.org/archives/list/capi-sig@python.org/thread/JPUNPN3AILGXOA3C2TTSLMOFNSWJE3QX/

See also my notes:
https://pythoncapi.readthedocs.io/optimization_ideas.html#tagged-pointers-doable

Wikipedia article: https://en.wikipedia.org/wiki/Tagged_pointer

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