python-libconfig: fix Python3 build

python-libconfig: fix Python3 build

boost_python library is named boost_python3, if boost is built
under Python3 environment. The patch fixes setup.py accordingly.

Fixes:
http://autobuild.buildroot.net/results/975/97533965180436c2f7a99de07fdc360ef57f84b0
http://autobuild.buildroot.net/results/b49/b49de32704f0f7ce5a610cf4363c6dcc2d8bafa1
http://autobuild.buildroot.net/results/e26/e26b4b9b486c582fb55826817a3428569968320f

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>