Add PEP 503 repository parsing

I'm looking at adding a new module, packaging.pep503 that will add utilities for parsing PEP 503 compliant repositories.

My biggest concern with doing this is that it requires a HTML5 parser. I'd plan on doing this by adding a dependency on html5lib, as there's no point in trying to write our own parser, but it's a pretty significant sized dependency (at least, compared to the packaging library itself). Is there any problem with adding html5lib as a dependency? Should we make it a soft dependency, only needed if the PEP 503 module is used, or is that too much complexity to be worth worrying about?

Start by reviewing PEP 503 and the proposed packaging.pep503 module. Investigate the html5lib dependency question and the project's existing dependency conventions before defining the parsing utilities. Done means the module's scope and dependency approach are agreed and the proposed repository parsing behavior is covered.