Javier Jardón authored Aug 24, 2018 and Tristan Van Berkom committed Aug 26, 2018

This fixes the following:
"
/home/jjardon/buildstream/buildstream/doc/bst2html.py:32: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  from collections import Mapping
"