scripts: install windows-curses package on Windows

The new menuconfig implementation needs it when run on Windows.

Use an environment marker to only install windows-curses on Windows. See
https://www.python.org/dev/peps/pep-0508/#id23

.

From some googling, sys_platform might be more widely supported than
platform_system, so use that.

Suggested-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>