Add an html5lib patch for Python 3.9 compatibility

[yan12125]

The patch is adapted from html5lib/html5lib-python@4f92357

Closes #6407
Closes #6237

---

Note that directly running invoke vendoring.update fails with: (excerpted logs)

    Running setup.py install for flit: started
      Running setup.py install for flit: finished with status 'error'
      Complete output from command /usr/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-mfhm_92g/flit/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-record-jsux_uqt/install-record.txt --single-version-externally-managed --prefix /tmp/pip-build-env-_123g61o/overlay --compile:
      Traceback (most recent call last):
        File "<string>", line 1, in <module>
      ModuleNotFoundError: No module named 'setuptools'

I need to apply #6606 to make vendoring work.

Also, invoke vendoring.update changes line endings from LF to CR/LF in src/pip/_vendor/pytoml/LICENSE. That seems irrevelant to html5lib, so I didn't commit that file.

[pradyunsg]

Note that directly running invoke vendoring.update fails with

Ah. I'm going to have an uneventful weekend.

[pradyunsg]

Thanks for this PR @yan12125!

[pradyunsg]

I'm good with this.

One minor change -- I'd prefer to use "html5lib-collections.vendor" as the name of the NEWS fragment.

[pradyunsg]

One minor change -- I'd prefer to use "html5lib-collections.vendor" as the name of the NEWS fragment.

Merging. I'll do this myself in a follow up.

[yan12125]

Merging. I'll do this myself in a follow up.

Thanks!