Replace html5lib with html5lib-modern

[ashleysommer]

Replace html5lib with html5lib-modern

This removes another source of six dependency.
Fixes #2749

Note however, this is not the last source of six sub-dependency.

The isodate module used in RDFLib also depends on six, and also hasn't been updated in over 3 years, (though it is not marked as deprecates like html5lib)

The path to fix, upgrade or replace isodate is not so clear.

[coveralls]

coverage: 90.649%. first build
when pulling fee0531 on html5lib-modern
into 64b778c on main.

[ashleysommer]

Update on the isodate issue, looks like that package is maintained by our colleague Gerhard at TERN, so he might be able to push out a new version with no six dependency.

[ashleysommer]

If there is no opposition, I will merge this on Monday.

[nicholascar]

Waiting to see if the new html5lib release removes six

[ashleysommer]

Waiting to see if the new html5lib release removes six

Actually I misinterpreted the situation. The original author has not put out a new html5lib release, and more projects are now moving to html5-modern.