import nt, sys; assert sys.executable.startswith(nt._getvolumepathname(sys.executable))

This code fails only when run from the python.bat as created by pcbuild\build.bat. The obvious difference is that the batch file sets PYTHONHOME which, presumably, is used to form sys.executable (haven't checked the startup code yet).

The docs for GetVolumePathName [*] don't specify that the drive letter of the path returned will be upper-case, but it doesn't seem unlikely.

So... it looks as though the test is unduly sensitive to case-differences in the face of something like PYTHONHOME which affects the way in which sys.executable is formed.

Phew! I'll put a test patch together later...

[*] https://docs.microsoft.com/en-us/windows/desktop/api/fileapi/nf-fileapi-getvolumepathnamew