The html5lib library isn't strictly required as the same functionality
can be achieved through the stdlib html.parser module.
The html5lib is one of the largest uses of the six library. By dropping
this unnecessary dependency, the pip project is closer to dropping the
six library.
Additionally, html5lib maintenance has slowed down and the project has
rejected pull requests to drop Python 2 support.
For now, the html5lib code remains, but is gated behind a command
line option: `--use-deprecated=html5lib`. After a sufficient amount of
time has passed without any reported bugs, the vendored library and this
flag can be removed completely.