Update conf.py to use packaging to detect dev vs release versions (#631)

# 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

---------

Co-authored-by: Grzegorz Bokota <[email protected]>

Author: psobolewskiPhD

docs/conf.py

@@ -34,12 +34,12 @@
 import napari
 from napari._version import __version_tuple__
 
-release = napari.__version__
-if "dev" in release:
+napari_version = parse_version(napari.__version__)
+release = str(napari_version)
+if napari_version.is_devrelease:
     version = "dev"
 else:
-    version = release
-
+    version = napari_version.base_version
 # -- Project information -----------------------------------------------------
 
 project = 'napari'
@@ -102,7 +102,7 @@
 if version == "dev":
     version_match = "dev"
 else:
-    version_match = release
+    version_match = str(release)
 
 html_theme_options = {
     "external_links": [