[frontend][python] avoid 'collections.abc' warnings
    
    Abstract classes were moved from 'collections' to
    'collections.abc' in Python 3, and accessing through 'collections'
    will stop work entirely in Python 3.8, original warning:
    
    /usr/lib/python3.7/site-packages/copr/client_v2/resources.py:3:
    DeprecationWarning: Using or importing the ABCs from 'collections'
    instead of from 'collections.abc' is deprecated, and in 3.8 it
    will stop working