Packaging: remove setup.cfg, move info into pyproject.toml

[GenevieveBuckley]

Closes #167

Now that pyproject.toml supports project metadata (classifiers, author info, etc.) we should remove the setup.cfg file and put the information into pyproject.toml

Advantages:

• simplifies the repository

• It makes it much easier for users to retro-fit napari plugins to existing

• it discourages people from accidentally duplicating info across both pyproject.toml and setup.cfg (if this happens pyproject.toml overwrites the information).

  For an in the wild example of this problem, Draga and I got an email from some users confused about why their plugin was not being picked up by the napari hub. It turned out that they had classifier tags in both pyproject.toml AND setup..cfg, and so the napari framework tag was not being included in the packaged build (and therefore not available on PyPI for the napari-hub to discover it)

[Czaki]

Did you need help with debug, or just do not have time?

[GenevieveBuckley]

This is ready for review

[Czaki]

As we dropped python 3.9 already, we may want to not suggest python 3.8 compatibility for plugins.