cmake: Fixed a bug where python 3.8 was not detected on Windows

Fixed a bug where python 3.8 was not detected on Windows.

Python 3.8 was released two weeks ago and any Windows user that
installs it will detect the wrong version.

Also add detection for 3.9 for future-proofing.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>