drop six dependency (#618)

[willkg]

This does some squirrely things in order to drop the six dependency that our vendored html5lib has. The thinking is along these lines:

1. this is the best way to drop the six dependency in the context we have
2. it maintains the rigor we use for vendored libraries
3. it's mostly straightforward and easy to reason about
4. the code changes are minimal--it mostly teaks a small handful of import lines
5. it won't cause problems with other things that are installed alongside bleach and its vendored html5lib
6. html5lib hasn't had a release in a while so this won't meaningfully increase the maintenance burden

The way this works is that vendoring now has two steps:

1. installing html5lib 1.1 into a specific directory with no dependencies
2. applying the 01_html5lib_six.patch

Fixes #618.

[willkg]

I don't need this. I'll fix it.