PEP 739: Static description file for build details of Python installations

Okay, PEP author has spoken. Please include it in Rejected Ideas though - there was enough discussion/interest that it’ll probably be brought up again if not.

I expect both of these to be true anyway. Certainly in the “preparing an install for deployment” case (where a venv usually won’t work anyway), having a venv-specific environment-details.json isn’t going to prevent anyone from having to special-case it.