DeprecationWarning from _vendor/html5lib

[belm0]

Environment

• pip version: 19.0.3
• Python version: 3.7.1

Description
pip/_vendor/html5lib/_trie/_base.py is emitting a DeprecationWarning and apparently the code will stop working in Python 3.8.

Expected behavior
pip runs on any Python version without emitting Python deprecation warnings

How to Reproduce
This problem came up via internal pip package usage by isort (PyCQA/isort#924). I've suggested that isort migrate to pip-api, but in any case it seems pip will have to address the impending code breakage of its internals.

Output

.../python3.7/site-packages/pip/_vendor/html5lib/_trie/_base.py:3: in <module>
    from collections import Mapping

E           DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working

[uranusjr]

Duplicate to #6237