CI: Switch to pypa/gh-action-pypi-publish@release/v1

Resolve the following warning:

  Warning:  You are using "pypa/gh-action-pypi-publish@master". The
  "master" branch of this project has been sunset and will not receive
  any updates, not even security bug fixes. Please, make sure to use a
  supported version. If you want to pin to v1 major version, use
  "pypa/gh-action-pypi-publish@release/v1". If you feel adventurous, you
  may opt to use use "pypa/gh-action-pypi-publish@unstable/v1" instead.
  A more general recommendation is to pin to exact tags or commit shas.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

.github/workflows/ci.yaml

@@ -80,7 +80,7 @@ jobs:
         run: python -m build --sdist --wheel --outdir dist/ .
       - name: Publish distribution to PyPI
         if: startsWith(github.ref, 'refs/tags')
-        uses: pypa/gh-action-pypi-publish@master
+        uses: pypa/gh-action-pypi-publish@release/v1
         with:
           password: ${{ secrets.PYPI_API_TOKEN }}
       - name: Create release on GitHub