Add specification for dependency specifiers

[konstin]

This moves PEP 508 to packaging.python.org as discussed on discord

Diff between PEP 508 and dependency-specifiers.rst: https://gist.github.com/konstin/059c0e72ff77049e0eb2ce0d738990fd. I've removed the PEP-specific paragraphs, added an introductory sentence and s/this PEP/this document/g.

[bhrutledge]

@konstin Thanks for this. I don't feel qualified to review this, and I don't follow Discord. Do you have a sense of who from that discussion would be a good reviewer? And/or, can you provide a link to that discussion?

[konstin]

Either @brettcannon or @pradyunsg probably

[konstin]

I got a CI failure email for https://github.com/pypa/packaging.python.org/actions/runs/3629838663/jobs/6122501649 but unfortunately i don't understand how this relates to this PR or why it failed (all checks on this PR seem to pass?)

[bhrutledge]

I got a CI failure email for https://github.com/pypa/packaging.python.org/actions/runs/3629838663/jobs/6122501649 but unfortunately i don't understand how this relates to this PR or why it failed (all checks on this PR seem to pass?)

Yeah, not sure what's going on here; see #1178. FYI @webknjaz

[brettcannon]

I'm assuming this is mostly a wholesale copying of PEP 508, so I'm reviewing it just from a perspective of not losing key details from what was previous here.

[konstin]

I'm assuming this is mostly a wholesale copying of PEP 508, so I'm reviewing it just from a perspective of not losing key details from what was previous here.

yes, i've included the gist link at the top so you can review the exact section-by-section differences from the document vs. the original PEP 508.

[pradyunsg]

Let's do this and we can iterate on this as we see fit. :)