Update vendored copy of html5lib for Python 3.8 compatibility

[tirkarthi]

Environment

• pip version: 19.0
• Python version: 3.8.0
• OS: Linux

Description

The vendored copy of html5lib is incompatible with changes to be made in CPython regarding collections import in python/cpython#10596. This is fixed on master with html5lib/html5lib-python#402 and opening this as a reminder issue as per python/cpython#10596 (comment)

cc : @pradyunsg

Thanks

[pradyunsg]

We'll handle this once the release is made by html5lib. :)

[gaborbernat]

Blocked by html5lib/html5lib-python#419

[zooba]

Since we know their next release (whenever it happens) will include this fix, can we just patch it in pip? I'm keen to upgrade the bundled version of pip for Python 3.8, but we really ought to do that sooner rather than later.

[pradyunsg]

Yep yep. Happy to hold a patch for this.

[yan12125]

Yep yep. Happy to hold a patch for this.

Glad to hear that one of major pip developers is fine with a patch. I created #6728 to fix the issue.