use/link to well known labels for `project.urls`

[sneakers-the-rat]

currently we have the following for project.urls:

python-package-guide/tutorials/pyproject-toml.md

Lines 566 to 568 in 9d064f3

	"Homepage" = "https://www.pyopensci.org"
	"Bug Reports" = "https://github.com/pyopensci/pyosmeta/issues"
	"Source" = "https://github.com/pyopensci/pyosmeta/"

I would propose that we use values from the pypa well-known labels list, and keep them lower snake case like the rest of the pyproject.toml file for consistency's sake - a string key in toml is not something i think we discuss? (could be wrong):

• homepage
• issues (or bugtracker, closest to "Bug Reports")
• repository (or source, either way)

We also have a link to an unofficial demo repo here:

python-package-guide/tutorials/pyproject-toml.md

Line 572 in 9d064f3

There are many other urls that you can add here. Check out the [README file here for an overview](https://github.com/patrick91/links-demo).

that has a bunch of extended examples which are nice (and i didn't know about some of them!), but we should also link to the pypa docs as well: https://packaging.python.org/en/latest/specifications/well-known-project-urls/