[3.13] gh-126433: Change channel_info.count to int64_t (GH-126447) (#126826)

gh-126433: Change channel_info.count to int64_t (GH-126447)

Fix compiler warnings on 32-bit Windows: change channel_info.count
type from Py_ssize_t to int64_t in _interpchannelsmodule.c.
(cherry picked from commit 09d7083962062acfef7e7a9a309a01fb70ad8276)

Co-authored-by: Victor Stinner <vstinner@python.org>