Use `pyproject.toml` to handle versioning

.pre-commit-config.yaml

Makefile

VERSION

docs/contents/workers/ci/index.md

docs/contents/workers/template-structure.md

pyproject.toml

setup.py

worker-{{cookiecutter.slug}}/Dockerfile

worker-{{cookiecutter.slug}}/MANIFEST.in

worker-{{cookiecutter.slug}}/Makefile

worker-{{cookiecutter.slug}}/VERSION

worker-{{cookiecutter.slug}}/pyproject.toml

worker-{{cookiecutter.slug}}/setup.py