Use poetry for dependency and packaging tooling

[Ichimonji10]

Drop setuptools and the files it depends upon, such as setup.py and setup.cfg, in favor of poetry.

Some apparent bugs were fixed in the process:

• pyproject.toml depends upon prometheus-client = "^0.7.1", but setup.py depends upon prometheus_client==0.7.1.
• pyproject.toml depends upon python = "^3.6", but setup.py depends upon python>=3.6.
• pyproject.toml listed far more dependencies than setup.py.
• pyproject.toml listed both wait-for and wait_for as dependencies.
• pyproject.toml lacked a description.

Fix: #94