Return metadata in setup

[dabacon]

Since version 1.3 of sphinx, the setup can return a dictionary with meta data about the extension. The details of this metadata are https://www.sphinx-doc.org/en/master/extdev/index.html

This PR adds this metadata. Importantly this stops a warning from sphinx that the app does not return whether it is parallel (read/write) safe. My understanding of this app is that it is parallel safe for both, so returned True for both.

I rearranged the setting of __version__, but one could also import _version and use directly.

[asmeurer]

Looks good. We'll have to do some clean up of the CI to release.