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

.gitignore

@@ -1,3 +1,4 @@
+.cache/
 *.egg-info/
 .python-version
 .venv/