Add PyPI credentials

Possibly there's a way to have it pick these up from the environment, but for now we can try what py-quantaq does: https://github.com/quant-aq/py-quantaq/blob/166f63b748d6267de2a744b0429bb8742ac08949/.github/workflows/release-to-pypi.yml#L31C32-L31C95

.github/workflows/release-to-pypi.yml

@@ -20,4 +20,4 @@ jobs:
       - uses: pdm-project/setup-pdm@v4
 
       - name: Publish package distributions to PyPI
-        run: pdm publish
+        run: pdm publish --user ${{ secrets.PYPI_USERNAME }} --password ${{ secrets.PYPI_PASSWORD }}