Centos has 3.6 natively, hence pip will install into sysroot/3.6 folder.
But poky 2.7+ uses python 3.7, hence doesn't looks into 3.6 folder. Mind
when using docker, polib was installed by the host, creating the
container.  Hence the problem occurs when boostrapping the buld on a
native Centos system.
Mind that this means we have to update polib manually upon upgrades