Use pyproject and remove setup.py

[edsu]

This commit includes changes to remove the use of setup.py and setuptools in favor of using the existing pyproject.toml and hatch.

A source layout was chosen, which required several changes such as:

• pushing bagit.py into src/bagit/
• pushing locale into src/bagit/locale

The gettext related code in setup.py was relocated to utils/locales.py It's very possible the way that the gettext functionality is setup needs some adjustment, as I'm new to using it.

Also we now rely on running tests with uv and pytest, which happens in the test github action. We could chose to update the PyPI publishing step to use uv build/publish too?