Modernise build tooling and configuration

[MarkBird]

Transition the project from the old-school setup.py style to the more modern pyproject.toml approach.

[dabapps-robot]

Nice update overall — this is a solid modernization of the packaging/release setup.

High-level

Moving from setup.py to pyproject.toml + python -m build and switching to pypa/gh-action-pypi-publish is a great simplification. The metadata looks clean, and removing the old custom packaging helpers is definitely a maintenance win.

One thing to double-check

• __version__ removal from enforce_host.__init__
  This may be a small backwards-incompatible change for anyone doing from enforce_host import __version__.
  If that’s considered part of the public API, it may be worth keeping it (or adding a deprecation path) even though version metadata now lives in pyproject.toml.

If dropping __version__ is intentional, I think this PR is otherwise in good shape to merge.