bpo-43284: Update platform.win32_ver to use _syscmd_ver instead of sys.getwindowsvers...

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.
(cherry picked from commit 2a3f4899c63806439e5bcea0c30f7e6a6295a763)

Co-authored-by: Shreyan Avigyan <shreyan.avigyan@gmail.com>