build(deps): bump actions/download-artifact from 3 to 4 (#30)

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 3 to 4.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v3...v4)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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Author: dependabot[bot]

.github/workflows/build-n-publish-to-pypi.yml

@@ -54,7 +54,7 @@ jobs:
     if: github.repository_owner == 'insarlab' && github.event_name == 'push'
     steps:
 
-      - uses: actions/download-artifact@v3
+      - uses: actions/download-artifact@v4
         with:
           # unpacks default artifact into dist/
           # if `name: artifact` is omitted, the action will create extra parent dir