Solve deprecation warning for collections.abc

[prcr]

Solve the deprecation warning for collections.abc:

$ mkdocs build
INFO     -  DeprecationWarning: Using or importing the ABCs from 'collections' instead of from
            'collections.abc' is deprecated since Python 3.3, and in 3.10 it will stop working
              File "/usr/lib/python3/dist-packages/html5lib/_trie/_base.py", line 3, in <module>
                from collections import Mapping
              File "/usr/lib/python3.8/collections/__init__.py", line 49, in __getattr__
                warnings.warn("Using or importing the ABCs from 'collections' instead "
INFO     -  Cleaning site directory

[prcr]

I'm closing this issue since a newer version of html5lib has already been released that fixes the issue: html5lib/html5lib-python#419