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

[vstinner]

PyConfig_Set() and sys.set_int_max_str_digits() now replace sys.flags, 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.

• Issue: Data race in sys_set_flag when sys.set_int_max_str_digits() is called concurrently with free-threaded build #151218

[vstinner]

I built Python with ./configure --with-pydebug --disable-gil --with-thread-sanitizer CC=clang LD=clang and I ran the reproducer: TSan doesn't report data races anymore.

If we agree that this change is the right approach, I will add a non-regression test.

[skirpichev]

I think it's a correct approach.

[vstinner]

I added a thread test to test_free_threading.test_sys which is run by ./python -m test --tsan.

I also added some simple tests on get_int_max_str_digits() / set_int_max_str_digits() (in test_sys).

[vstinner]

@picnixz @gpshead: Would you mind to review this fix?

[vstinner]

Python 3.13 is not affected: sys.set_int_max_str_digits() only sets tstate->interp->long_state.max_str_digits, it doesn't update sys.flags. And PyConfig_Set() was only added to Python 3.14.

I modified sys.set_int_max_str_digits() in Python 3.14 when I implemented PEP 741 (PyConfig_Set()).