BLD: specify build-system and move project metadata to pyproject.toml

[mwtoews]

This refactor moves project metadata to pyproject.toml as defined by PEP 621, and is similar to cylc/cylc-flow#7279 and cylc/cylc-uiserver#895

Firstly, this now specifies a build-system PEP 517 based on setuptools, done at the top of pyproject.toml.

As for metadata translation, this is intended to be as close to a 1:1 conversion with minimal other changes, with a few exceptions or notes:

• Same authors as cylc-flow
• Expand URLs, again similar to cylc-flow's URL metadata; add Documentation link
• Clarify GPL licence to SPDX-compatible short identifier: GPL-3.0-or-later
• Specify licence files: ["COPYING", "CONTRIBUTING.md"]

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.