gh-91181: drop support for bytes on sys.path (GH-31934)

Support for bytes broke sometime between Python 3.2 and 3.6 and has been broken ever since. Trying to bring back supports is surprisingly difficult in the face of -b and checking for keys in sys.path_importer_cache. Since the support was broken for so long, trying to overcome the difficulty of bringing back the support has been deemed not worth it.

Co-authored-by: Eryk Sun <eryksun@gmail.com>
Co-authored-by: Brett Cannon <brett@python.org>