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 .
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.