build!: requirements/edx-sandbox/py38.txt (#36049)

This was a temporary backcompat alias to requirements/edx-sandbox/quince.txt.

Developers relying on this file to run a python3.8 sandbox should:
* first, switch to requirements/edx-sandbox/quince.txt, which works with
* python3.8 and then, upgrade their codejail sandbox to a newer `<release>.txt`
* since python3.8 is EOL.

See the readme in this directory for more details.