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

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

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>