🎨 Convert action inputs to use kebab-case

Up until now, the action input names followed the snake_case naming
pattern that is well familiar to the pythonistas. But in GitHub
actions, the de-facto standard is using kebab-case, which is what
this patch achieves.
This style helps make the keys in YAML better standardized and
distinguishable from other identifiers.
The old snake_case names remain functional for the time being and will
not be removed until at least v3 release of this action.

.github/workflows/self-smoke-test-action.yml

@@ -91,7 +91,7 @@ jobs:
       with:
         user: ${{ env.devpi-username }}
         password: ${{ env.devpi-password }}
-        repository_url: >-
+        repository-url: >-
           http://devpi:${{ env.devpi-port }}/${{ env.devpi-username }}/public/
 
 ...