Vendored libs warning about from collections import <Abstract> instead of collections.abc

[aaronchall]

Vendored libs warning about from collections import <Abstract> instead of collections.abc

This is an issue in both

• bleach/bleach/_vendor/html5lib/_trie/_base.py
  from collections import Mapping
  should be
  from collections.abc import Mapping

and

• bleach/bleach/_vendor/html5lib/treebuilders/dom.py
  from collections import MutableMapping
  should be
  from collections.abc import MutableMapping

=============================== warnings summary ===============================
/nix/store/2xz5xcz568v9vdasqf9p45w97znkm8pp-python3.8-bleach-3.1.0/lib/python3.8/site-packages/bleach/_vendor/html5lib/_trie/_base.py:3
  /nix/store/2xz5xcz568v9vdasqf9p45w97znkm8pp-python3.8-bleach-3.1.0/lib/python3.8/site-packages/bleach/_vendor/html5lib/_trie/_base.py:3: 
DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, 
and in 3.9 it will stop working
    from collections import Mapping

-- Docs: https://docs.pytest.org/en/latest/warnings.html

These are alluded to here as well: #425

[aaronchall]

#521