bpo-43284: Update platform.win32_ver to use _syscmd_ver instead of sys.getwindowsversion() (GH-25500)

The sys module uses the kernel32.dll version number, which can vary from the "actual" Windows version.
Since the best option for getting the version is WMI (which is expensive), we switch back to launching cmd.exe (which is also expensive, but a lot less code on our part).
sys.getwindowsversion() is not updated to avoid launching executables from that module.