Update conf.py to use packaging to detect dev vs release versions

[psobolewskiPhD]

References and relevant issues

If you go to dev docs right now:
https://napari.org/dev/index.html
The sidebar link to the release notes is broken, I think because the version is being treated as a release and not dev.
It points to:
https://napari.org/dev/release/release_0_6_0a0.html
which doesn't exist.
instead of the index.html which would be used for a dev release:
https://napari.org/dev/release/index.html

So in this PR I use packaging to check if a version is a dev version and also uses packaging to get just the base version number of pre-releases, so we will get release_0_6_0.html

[willingc]

Thank you! 🚢

[psobolewskiPhD]

Checking:
https://output.circle-artifacts.com/output/job/c902cc9c-bfe0-46f3-90de-6c69c2cecbdf/artifacts/0/docs/docs/_build/html/index.html
Looks like it worked

[willingc]

I'm not sure if it's caching or the deploy workflow but stable says (0.5.6) but directs to 0.6.0 🤷🏼‍♀️

[psobolewskiPhD]

Can you explain where? when I go to napari.org stable is 0.5.6 and has for example the missing screenshot in installation guide. When I use the circle build, it's just building dev, so it should also have stable as napari.org stable.

[Czaki]

We should deploy prerelease to separate dirctory. (for 0.6.0a1 it should be 0.6.0) but we should not parse it manually when packaging is available.

[Czaki]

Please update the title to reflect the current state

Comments were addressed + suggestions were accepted