Create a reusable workflow for Python package publishing

[woodruffw]

This is a prime candidate for a reusable workflow, since it boils down to:

      - uses: actions/checkout@v3

      - name: Set up Python
        uses: actions/setup-python@v4
        with:
          python-version: '3.x'

      - name: Build distributions
        run: make dist

      - name: Publish a Python distribution to PyPI
        uses: pypa/gh-action-pypi-publish@release/v1
        with:
          password: ${{ secrets.PYPI_TOKEN }}

This flow will be even nicer when PyPI supports reusable workflows for publishing, which is something we're working on.