socketmodule.c: use atomics to access `defaulttimeout` in free-threaded build

[colesbury]

Feature or enhancement

The defaulttimeout in socketmodule.c is shared, mutable state:

cpython/Modules/socketmodule.c

Line 551 in 9f983e0

PyTime_t defaulttimeout;

In the free-threaded build, we should use relaxed atomic operations to access defaulttimeout to avoid data races. This probably requires adding wrappers for 64-bit relaxed atomic load and stores to https://github.com/python/cpython/blob/main/Include/internal/pycore_pyatomic_ft_wrappers.h. Note that _PyTime_t is a typedef for int64_t.

For context, here is a similar change from nogil-3.12, but note that defaulttimeout is now part of the module state:

• colesbury/nogil-3.12@360a79cb88

Linked PRs

• gh-116616: Use relaxed atomic ops to access socket module defaulttimeout #116623