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

noxfile.py

@@ -0,0 +1,114 @@
+"""Configure nox for the isolated test and lint environments."""
+
+import os
+import tempfile
+from typing import Any
+
+import nox
+from nox.sessions import Session
+
+
+MAIN_PYTHON = '3.8'
+
+# Keep the project is forward compatible.
+NEXT_PYTHON = '3.9'
+
+# Paths with *.py files.
+SRC_LOCATIONS = 'noxfile.py', 'src/'
+
+
+# Use a unified .cache/ folder for all tools.
+nox.options.envdir = '.cache/nox'
+
+# All tools except git and poetry are project dependencies.
+# Avoid accidental successes if the environment is not set up properly.
+nox.options.error_on_external_run = True
+
+# Run only CI related checks by default.
+nox.options.sessions = (
+    'format',
+    'lint',
+    f'test-{MAIN_PYTHON}',
+    f'test-{NEXT_PYTHON}',
+)
+
+
+@nox.session(name='format', python=MAIN_PYTHON)
+def format_(session: Session):
+    """Format source files."""
+    _begin(session)
+
+
+@nox.session(python=MAIN_PYTHON)
+def lint(session: Session):
+    """Lint source files."""
+    _begin(session)
+
+
+@nox.session(python=[MAIN_PYTHON, NEXT_PYTHON])
+def test(session: Session):
+    """Test the code base."""
+    _begin(session)
+
+
+@nox.session(name='pre-commit', python=MAIN_PYTHON, venv_backend='none')
+def pre_commit(session: Session):
+    """Source files must be well-formed before they enter git."""
+    _begin(session)
+    session.notify('format')
+    session.notify('lint')
+
+
+@nox.session(name='pre-merge', python=MAIN_PYTHON, venv_backend='none')
+def pre_merge(session: Session):
+    """The test suite must pass before merges are made."""
+    _begin(session)
+    session.notify('test')
+
+
+def _begin(session: Session) -> None:
+    """Show generic info about a session."""
+    if session.posargs:
+        print('extra arguments:', *session.posargs)
+
+    session.run('python', '--version')
+
+    # Fake GNU's pwd.
+    session.log('pwd')
+    print(os.getcwd())
+
+
+def _install_packages(
+    session: Session, *packages_or_pip_args: str, **kwargs: Any
+) -> None:
+    """Install packages respecting the poetry.lock file.
+
+    This function wraps nox.sessions.Session.install() such that it installs
+    packages respecting the pinnned versions specified in poetry's lock file.
+    This makes nox sessions even more deterministic.
+
+    Args:
+        session: the Session object
+        *packages_or_pip_args: the packages to be installed or pip options
+        **kwargs: passed on to nox.sessions.Session.install()
+    """
+    if session.virtualenv.reuse_existing:
+        session.log(
+            'No dependencies are installed as an existing environment is re-used',
+        )
+        return
+
+    session.log('Dependencies are installed respecting the poetry.lock file')
+
+    with tempfile.NamedTemporaryFile() as requirements_txt:
+        session.run(
+            'poetry',
+            'export',
+            '--dev',
+            '--format=requirements.txt',
+            f'--output={requirements_txt.name}',
+            external=True,
+        )
+        session.install(
+            f'--constraint={requirements_txt.name}', *packages_or_pip_args, **kwargs,
+        )