This PR includes a workflow that validates that future PRs include
a changelog entry (unless the `skip-changelog` label is present on the
PR, or the PR is a dependabot PR).
Use 'poetry run towncrier create` to create entries for the changelog.
This ensures that any releases are fully tested before publication.
The workflow first builds the distribution files (sdist, wheel) before
using a deployment environment to publish these to PyPI, using the
GitHub actions OpenID support to authenticate with PyPI.
Tox manages test environments for all supported Python versions, as
well as linting and formatting tools. On GitHub, the test and lint
steps are kept as close as possible to the Makefile equivalents.
