Add Makefile

[cdcarter]

Add a Makefile to handle basic development lifecycle.

Commands:

• help -- list all available makefile commands, and also reference that setup.py poetry is in use
• clean -- to remove all nonsense from the repo, split into clean-build, clean-cache, clean-test
• dist -- create tarball, egg and/or wheel of the package (whatever is appropriate)
• lint -- run black & flake8
• test -- run pytest (in the currently active venv)
• test-all -- run tox (it will create its own venv)
• install -- install (in the currently active venv)
• tag -- create a signed tag of the current HEAD
• release -- publish a release on github of the current tag

see CumulusCI Makefile for examples.