Packaging: Finish project metadata in pyproject.toml

[lancetarn]

We should fill out maintainers with support@scoutapm.com email, probably. Also would like to have links to marketing site and github repo.

https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#about-your-project

[quinnmil]

These changes were wrapped into this PR: #24