html5lib on python3

[koenedaele]

When using the py3 versions of RDFlib, HTML5lib has to be installed manually. With the the py2 versions it's already there because it's included in setup.py. The py3 version adds it to tests_require, not to install_requires.

[gromgull]

probably fixed in 3032bff

[joernhees]

see #671