package/python-shutilwhich: also available in python3

When in python3, shutilwhich will simply use shutil.which.

Some packages (e.g. crossbar.io) unconditionally require shutilwhich,
even when they are running under python3.

So, enable python-shutilwhich even for python3.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Mauro Condarelli <mc5686@mclink.it>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>