I have no choice, because 3.9 isn't supported on Windows 7. I've always wondered what feature of Windows was so important that they had to drop compatibility with older versions, it's intentionally a very stable interface.
An unofficial Python installer for Windows 7 exists: https://github.com/adang1345/PythonWin7 (it uses an unofficial implementation of api-ms-win-core-path-l1-1-0.dll).
Have you tried building 3.9 from source to see if there's a particular API that's unsupported?
No, I haven't tried to build from source. That's a rabbit hole I'm not particularly interested in jumping down.