Remove `setup.py` (only need `pyproject.toml`)

[jcannon-gplates]

See discussion at #263.

In summary:

Due to 205872c, which uses setuptools-git-versioning:

• Git tags are being used to find the version when packaging (as described here).
  • And this explains why we get versions like 1.0.0.post1+git.64e68cd4 after a release.
  • That means we no longer need PYPI_VERSION in setup.py.
    • The only place, other than git tags, where the version is currently being updated.
• At run time, GPlately will get the version info from package's metadata.
  • Because we essentially do this: __version__ = importlib.metadata.version("gplately").
  • But the version info was originated from a git tag in the first place.

So we should be able to simply remove setup.py - pyproject.toml does everything it currently does.