package/setools: install for host-python3

package/setools: install for host-python3

setools needs python3 since version 4.2.0 and
https://github.com/SELinuxProject/setools/commit/e292a77c52ed882707c8d165e77914837367313f

However today in Buildroot, when no target python is selected, or when
BR2_PACKAGE_PYTHON=y, all host python modules are installed for
host-python, i.e Python 2. But this module won't install in Python 2,
so let's force its host variant to be installed with Python 3 on the
host. Of course, for that to work, its dependency must also be built
for host-python3, so we change it to the newly introduced
host-python3-cython package.

Fixes:
 - http://autobuild.buildroot.org/results/c2febcea5fbd8a21709721524ae9e9b5fc0896f9

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Tested-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>