Pylint: whitelist packages with extension modules
    
    Pylint refuses to load extension modules from unsafe places. This
    triggers import-error failures for pylint runs inside a tox virtualenv.
    Any module or package in extension-pkg-whitelist is whitelisted and
    pylint imports extension modules.
    
    https://fedorahosted.org/freeipa/ticket/6468
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>