cext: fix Python 3.11 compatibility - Py_SIZE is not an lvalue (issue6610)
Py_SIZE was made a static inline function during Python 3.10 development, as
described on https://vstinner.github.io/c-api-opaque-structures.html .
e92ca942ddca updated the Mercurial code base accordingly, but somehow missed a
couple of cases introduced long time ago in a8c948ee3668.
The Python change was dropped for 3.10, but is coming back again in 3.11 .
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