Drop Support for Python 2.7

[aqeelat]

I'd like to work on this

[gustavi]

Note: it will remove the six dependency (which would be a great news !)

[Mic92]

#580

[ashleysommer]

@aqeelat @gustavi

I've published my own fork release here: https://pypi.org/project/html5lib-modern/
It is the main branch v1.2-dev code as-of today, plus the "remove python2" changes plus the "drop six" changes.
It also includes a couple of tweaks of my own, including vendorizing webencodings library so we can include a modernised version of that, and this version of html5lib now has zero dependencies.
Additionally, the module is now packaged with a pyproject.toml file using the new PEP-517 pyPA build tooling.