[3.15] gh-151218: Replace sys.flags in PyConfig_Set() (GH-151402) (#151552)

[3.15] gh-151218: Replace sys.flags in PyConfig_Set() (GH-151402) (#151552)

gh-151218: Replace sys.flags in PyConfig_Set() (GH-151402)

PyConfig_Set() and sys.set_int_max_str_digits() now replace
sys.flags (create a new object), instead of modifying sys.flags in-place.

Modifying sys.flags in-place can lead to data races when multiple
threads are reading or writing sys.flags in parallel.

Use _Py_atomic functions to get and set max_str_digits members.
(cherry picked from commit b16d23fc9fe9cb72fa15c8a3036753e5437b5b8c)

Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>