gh-116738: make entering of contextvars.Context thread safe

[kumaraditya303]

• Issue: Audit all built-in modules for thread safety #116738

[colesbury]

ctx->ctx_entered functions like a lock. So I think I'd write this as:

    int already_entered = FT_ATOMIC_LOAD_INT_RELAXED(ctx->ctx_entered);
    if (!already_entered) {
        ...
    }
    if (ts->context != (PyObject *)ctx) {
        ...
    }
    FT_ATOMIC_STORE_INT_RELEASE(ctx->ctx_entered, 0);

[kumaraditya303]

Thanks! I used FT_ATOMIC_STORE_INT as there is no FT_ATOMIC_STORE_INT_RELEASE and I don't think using seq-cst over release would hurt performance here.