The version string in the `__init__` module is needed to support
towncrier, but that does mean there are now two locations for the
project version: pyproject.toml and the `__init__.py` file. Use
`importlib.metadata` to pull the version from the installation metadata.
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.
