package/python3-decorator: new package

The host-python-networkx package needs to be built for the
host-python3, even when the target Python is not necessarily Python
3.x. Since it depends on host-python-decorator, we need a Python 3
variant of it, which this patch introduces.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>