[3.12] gh-105059: Fix MSCV compiler warning on PyObject union (GH-107239) (#107248)

gh-105059: Fix MSCV compiler warning on PyObject union (GH-107239)

Use pragma to ignore the MSCV compiler warning on the PyObject
nameless union.
(cherry picked from commit 1c8fe9bdb624d356643ee569151a9e4f2963179a)

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