Only trigger publish on release.published event (#572)

We noticed that making a release triggered
multiple different release events: created, prereleased, published. 
Here we tweak the workflow file so that the workflow only runs
for the release.publish event, per documentation:
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#release

.github/workflows/publish.yml

@@ -5,9 +5,11 @@ on:
     pull_request:
         branches:
         # Commits pushed to release/ branches are published on Test PyPI if they
-        # have a new version number.
+        # have a new version number. This allows the maintainer to check the release
+        # before it is pushed to the actual PyPI index.
             - release/**
     release:
+        types: [published]
 
 jobs: