west sign: prefer 'python imgtool.py' on windows

We can't trust that a python file is executable on Windows, regardless
of what the mode bits say.

When we find that imgtool is a .py file, run 'python imgtool.py'
instead of 'imgtool.py' on that platform.

Fixes: #31876
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>