python: release GIL around hy_sack_load_yum_repo().
python: release GIL around hy_sack_load_yum_repo().

This is mainly for Anaconda, which is a bit threading-heavy. Without this
hy_sack_load_yum_repo(), which is probably the single most expensive hawkey
operation, starves the other threads (most painfully the UI).

mizdebsk/hawkey,cgwalters/hawkey,ignatenkobrain/hawkey,jsilhan/hawkey,megaumi/hawkey,MichaelMraka/hawkey,ignatenkobrain/hawkey,mluscon/hawkey,ignatenkobrain/hawkey,cgwalters/hawkey,mluscon/hawkey,Conan-Kudo/hawkey,mluscon/hawkey,cgwalters/hawkey,Conan-Kudo/hawkey,jsilhan/hawkey,jsilhan/hawkey,rholy/hawkey,Conan-Kudo/hawkey,yavor-atanasov/hawkey,MichaelMraka/hawkey,MichaelMraka/hawkey,ignatenkobrain/hawkey,mizdebsk/hawkey,rholy/hawkey,rholy/hawkey,rpm-software-management/hawkey,mizdebsk/hawkey,yavor-atanasov/hawkey,cgwalters/hawkey,rpm-software-management/hawkey,rpm-software-management/hawkey,jsilhan/hawkey,Conan-Kudo/hawkey,MichaelMraka/hawkey,yavor-atanasov/hawkey,rpm-software-management/hawkey,yavor-atanasov/hawkey,megaumi/hawkey,megaumi/hawkey,mizdebsk/hawkey,mluscon/hawkey,rholy/hawkey,megaumi/hawkey