This commit creates a flake.nix file defining a minimal build
environment for Nix users. If you have Nix, you can run

    nix develop

to enter a FHS-compliant environment with Python 3.10, 3.11, 3.12,
3.13, and 3.14 installed, along with development tools like tox and
pre-commit. From here, ordinary commands like

    tox -e linting,py310,py311,py312,py313,py314

work as expected.

Note that this environment does not provide any Python packages except
tox; all dependencies are installed via tox as if we were not using
Nix.