Add nox as the task runner

- set up skeletons for all planned nox sessions
- provide a base configuration for all nox sessions
- create two utility functions:
  + _begin() => run commands common to all sessions
  + _install_packages() => install dependencies in nox sessions
                           respecting the versions pinned in poetry.lock
- add the following file:
  + noxfile.py

pyproject.toml

@@ -23,3 +23,5 @@ repository = "https://github.com/webartifex/urban-meal-delivery"
 python = "^3.8"
 
 [tool.poetry.dev-dependencies]
+# Task Runners
+nox = "^2020.5.24"