Make html2vimdoc Python 3 compatible

[blueyed]

I am willing to work on that - but maybe there is something done in this regard already?

Also porting to BeautifulSoup 4 needs to be done apparently.

[mipmip]

This fork https://github.com/micbou/vim-tools has html2vimdoc working with python3. Maybe @micbou likes to create a PR. Install sections is also updated in the README. Vimdoctool.py gives errors.