package/gnuradio: force python3-six
host-python-six needs host-python (aka python2) but since
commit e9df5b0fc3f6e8bb36deedefb392bbc968116ee7 python-mako is
python3 only. python-six is build using python2 and python-mako with
python3 and gnuradio build fails with:
-- Python checking for python >= 2.7
-- Python checking for python >= 2.7 - found
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>