plugins: Don't treat keys of api as bytes
    
    The plugin `plugins` iterates over the keys of API instance,
    __iter__ of which is a generator of class.__name__ from
    (Command, Object, Method, Backend, Updater). So, the allowed type
    is str, not bytes.
    
    Fixes: https://pagure.io/freeipa/issue/8898
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>