gh-36437: src/sage/parallel/map_reduce.py: replace tmp_dir()
Standard `tempfile.TemporaryDirectory()` replacement for `tmp_dir()` and
fix a deprecation warning while we're at it.
Issue: https://github.com/sagemath/sage/issues/36322
URL: https://github.com/sagemath/sage/pull/36437
Reported by: Michael Orlitzky
Reviewer(s): Marc Mezzarobba
