README: fix Python package names in dependencies
README: fix Python package names in dependencies

With Fedora's switch to Python 3, all Python 2 packages should have
python2- prefix.

Conan-Kudo/libhif,rpm-software-management/libhif,Conan-Kudo/libhif,rpm-software-management/libdnf,rpm-software-management/libdnf,rpm-software-management/libdnf,rpm-software-management/libhif,hughsie/libhif,rpm-software-management/libdnf,rpm-software-management/libhif,hughsie/libhif,Conan-Kudo/libhif,Conan-Kudo/libhif,hughsie/libhif,rpm-software-management/libhif,hughsie/libhif