Add missing libhandy requires
    
    Fixes the following traceback when starting hydrapaper flatpak:
    
    Traceback (most recent call last):
      File "/app/bin/hydrapaper", line 57, in <module>
        gi.require_version('Handy', '0.0')
      File "/usr/lib64/python3.7/site-packages/gi/__init__.py", line 129, in require_version
        raise ValueError('Namespace %s not available' % namespace)
    ValueError: Namespace Handy not available