Release infrastructure update

The action ``pypa/gh-action-pypi-publish@master`` no longer works, need to replace with ``pypa/gh-action-pypi-publish@release/v1``.

Looks like the upload/download actions should be updated to v2 (``actions/download-artifact@v2``) as well.

This will need to be done across the tskit-dev projects, or the next release will fail to upload the wheels.