py: raise AttributeError when a checksum is not present.
py: raise AttributeError when a checksum is not present.

Previously this assert()ed.

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