Add workflow to publish releases to PyPI

[mdickinson]

This PR adds a workflow which automatically publishes releases to PyPI.

This is almost identical to the corresponding workflow on Envisage (which ran successfully this morning). The only difference is an extra line to upgrade pip if necessary. (This line doesn't make a difference right now, but it's not inconceivable that some future release of build or twine can only be installed properly with an updated pip.)

I've tested the commands in the workflow manually by:

• Creating a new, clean Python 3.10 venv
• Running python -m pip install --upgrade pip and python -m pip install build twine
• Running python -m build followed by python -m twine check --strict dist/*.

The strict checks passed for me.

[mdickinson]

Note: appropriate PYPI_USERNAME and PYPI_PASSWORD secrets have been added to the repository.