add workflow to publish to PyPI for new tags

[prjemian]

Per the publishing guidelines, to get a new version published on conda-forge, first must push that new version to PyPI. Then the conda-forge auto-tick bot will hopefully open a PR once it finds the new release on PyPI.

Need to add the workflow that publishes to PyPI on new tags.

[prjemian]

Create an access token on PyPI:

• name: GitHub Actions CI/CD — bluesky/hklpy
• scope: hklpy
• permissions: Upload packages.

Copy and add it (to this repo) with name PYPI_API_TOKEN as a repository secret.