Set up nox as the task runner

- base configuration for all nox sessions to come
- add infos about nox in the README file
- ignore [py]cache folders in git

.gitignore

@@ -1 +1,3 @@
+.cache/
+**/__pycache__/
 .venv/

README.md

@@ -45,6 +45,26 @@ To replicate the project maintainer's develop environment, run:
 `poetry install`
 
 
+### Maintenance Tasks
+
+We use [nox](https://nox.thea.codes/en/stable/) to run
+    the test suite and other maintenance tasks during development
+    in isolated environments.
+`nox` is similar to the popular [tox](https://tox.readthedocs.io/en/latest/).
+It is configured in the
+    [noxfile.py](https://github.com/webartifex/lalib/blob/main/noxfile.py) file.
+`nox` is assumed to be installed as well
+    and is therefore not a project dependency.
+
+To list all available tasks, called sessions in `nox`, simply run:
+
+`nox --list` or `nox -l` for short
+
+To execute all default tasks, simply invoke:
+
+`nox`
+
+
 ### Branching Strategy
 
 The branches in this repository follow the

noxfile.py

@@ -0,0 +1,92 @@
+"""Maintenance tasks run in isolated environments."""
+
+import collections
+from collections.abc import Mapping
+from typing import Any
+
+import nox
+from packaging import version as pkg_version
+
+
+try:
+    from nox_poetry import session as nox_session
+except ImportError:
+    nox_session = nox.session
+
+
+def nested_defaultdict() -> collections.defaultdict[str, Any]:
+    """Create a multi-level `defaultdict` with variable depth.
+
+    The returned `dict`ionary never raises a `KeyError`
+    but always returns an empty `dict`ionary instead.
+    This behavior is occurs recursively.
+
+    Adjusted from: https://stackoverflow.com/a/8702435
+    """
+    return collections.defaultdict(nested_defaultdict)
+
+
+def defaultify(obj: Any) -> Any:
+    """Turn nested `dict`s into nested `defaultdict`s."""
+    if isinstance(obj, Mapping):
+        return collections.defaultdict(
+            nested_defaultdict,
+            {key: defaultify(val) for key, val in obj.items()},
+        )
+    return obj
+
+
+def load_pyproject_toml() -> collections.defaultdict[str, Any]:
+    """Load the contents of the pyproject.toml file.
+
+    The contents are represented as a `nested_defaultdict`;
+    so, missing keys and tables (i.e., "sections" in the .ini format)
+    do not result in `KeyError`s but return empty `nested_defaultdict`s.
+    """
+    return defaultify(nox.project.load_toml("pyproject.toml"))
+
+
+def load_supported_python_versions(*, reverse: bool = False) -> list[str]:
+    """Parse the Python versions from the pyproject.toml file."""
+    pyproject = load_pyproject_toml()
+    version_names = {
+        classifier.rsplit(" ")[-1]
+        for classifier in pyproject["tool"]["poetry"]["classifiers"]
+        if classifier.startswith("Programming Language :: Python :: ")
+    }
+    return sorted(version_names, key=pkg_version.Version, reverse=reverse)
+
+
+SUPPORTED_PYTHONS = load_supported_python_versions(reverse=True)
+MAIN_PYTHON = "3.12"
+
+SRC_LOCATIONS = ("./noxfile.py", "src/")
+
+
+nox.options.envdir = ".cache/nox"
+nox.options.error_on_external_run = True  # only `git` and `poetry` are external
+nox.options.reuse_venv = "no"
+nox.options.sessions = (  # run by default when invoking `nox` on the CLI
+)
+nox.options.stop_on_first_error = True
+
+
+def start(session: nox.Session) -> None:
+    """Show generic info about a session."""
+    if session.posargs:
+        session.debug(f"Received extra arguments: {session.posargs}")
+
+    session.debug("Some generic information about the environment")
+    session.run("python", "--version")
+    session.run("python", "-c", "import sys; print(sys.executable)")
+    session.run("python", "-c", "import sys; print(sys.path)")
+    session.run("python", "-c", "import os; print(os.getcwd())")
+    session.run("python", "-c", 'import os; print(os.environ["PATH"])')
+
+    session.env["PIP_CACHE_DIR"] = ".cache/pip"
+    session.env["PIP_DISABLE_PIP_VERSION_CHECK"] = "true"
+
+
+if MAIN_PYTHON not in SUPPORTED_PYTHONS:
+    msg = f"MAIN_PYTHON version, v{MAIN_PYTHON}, is not in SUPPORTED_PYTHONS"
+    raise RuntimeError(msg)