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]

Move this to test_free_threading/test_str.py and remove the @threading_helper.reap_threads -- it's not needed

[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;
}

[colesbury]

Fix a crash in the free-threaded build when the dictionary argument to
:meth:`str.maketrans()` is concurrently modified.

We don't need to include implementation details about the critical section in the NEWS blurb.

[colesbury]

Just for _ in range(5):. Same with the loop below. Don't bother to assign the iteration count to local variables.