feat: migrate remainder of pyproject.toml to new-style

Without specifying vcs-ref and when the copier template is a git clone (as it is in CI), copiers default behaviour is to look for the most recent version-number-tagged git commit and copy from that.

The upshot of this is that we've, for quite some time, been testing the most recent releases in CI jobs and never actually testing the changes!

Poetry additionally understands the new-style "project.name", etc fields so migrate those as well as "requires-python".