gh-145142: Make str.maketrans safe under free-threading

[VanshAgarwal24036]

Replace unsafe PyDict_Next iteration with snapshot iteration using PyDict_Items to avoid crashes when the input dict is mutated concurrently under free-threading.

Adds a regression test.

• Issue: str.maketrans on a mutating dict is not thread-safe #145142

[colesbury]

A few comments below

[colesbury]

Use a critical section here instead of copying the items to a list. You'll need to fix up the goto err. Something like:

{
    ...
    Py_BEGIN_CRITICAL_SECTION(x);
    while (PyDict_Next(x, &i, &key, &value)) {
        ...
    }
    goto done;
err:
    Py_CLEAR(new);
done:
    Py_END_CRITICAL_SECTION();
    return new;
}

[VanshAgarwal24036]

• It fails the Windows _freeze_module build (syntax errors from macro expansion).
• A version with early returns builds but causes test_str failures under free-threading on macOS/Linux

Is there a preferred pattern for using Py_BEGIN_CRITICAL_SECTION here, or would you prefer using the dict’s internal locking helpers instead?