remove six from dependencies

[sevdog]

Since support for Python 2 was removed in version 2.0 the use of SIX library to handle compatibility between Python 3 and Python 2 has no more meaning (aka: it is a useless dependecy).

See remaining code using SIX.

[g-k]

Happy to accept a PR for this and I'll leave this open to track the issue.

Keep in mind we don't want to edit the vendored html5lib since we'd effectively fork it. Last I checked this would require an upstream release of html5lib.

[sevdog]

Now that you mention it, this could be a problem since six is only used in vendored html5lib 🥲

Before rebasing that folder this PR should be merged by them.

[willkg]

I put this in the 5.0.0 milestone because I thought we could do it, but we can't, yet. We're blocked on upstream dropping the need for six.

[willkg]

It seems like upstream is no longer maintained. The PR in question was closed. Issue #680 covers forking html5lib or finding an alternative.