[PATCH stable] cext: fix Python 3.11 compatibility - Py_SIZE is not an lvalue (issue6610)

[Yuya Nishihara]

On Thu, 18 Nov 2021 16:49:28 +0100, Mads Kiilerich wrote:
> # HG changeset patch
> # User Mads Kiilerich <mads at kiilerich.com>
> # Date 1637235097 -3600
> #      Thu Nov 18 12:31:37 2021 +0100
> # Branch stable
> # Node ID 9c41494c5dd2fa8816bc7b70e76b28c4d3941af0
> # Parent  6e576e4665f43101ef9bb98b0f5234f85c5db2ea
> cext: fix Python 3.11 compatibility - Py_SIZE is not an lvalue (issue6610)

Queued for stable, thanks.