Split tags into a separate package

[brettcannon]

@steve.dower

The use of pyparsing seems more legitimate though. That code would be significantly more complex without it

It could be solved with a hand-written recursive-descent parser for PEP 508. I don’t think the grammar is too bad; honestly the URL parsing bit is way more than what PEP 508 introduces.

Not that I’m specifically volunteering to write said parser.

@agronholm

As for pyparsing, the ideal solution in my opinion would be the ability to install packaging without that dependency. That would require that the dependency be placed in extras, and that this extra would be installed by default. Right now we don’t have this capability.

I think this ties into Adding a default extra_require environment.