replace pkg_resources for python 3.12

[branchv]

Fixes #602

Replaces pkg_resources (deprecated, no longer bundled with Python since 3.12) with importlib following the migration guide:

• pkg_resources.resource_filename is now importlib.files
• The later returns a Traversable (a limited pathlib.Path API) rather than a file guaranteed to be on disk (i.e. for a zipapp). So we need to walk the (possibly virtual) directories ourselves using iterdir and read the files using read_bytes.

Also, I took the opportunity to migrate to pyproject.toml, the modern replacement for setup.py favoring static metadata (also replacing the need for pytest.ini and setup.cfg). This required:

• Adding build to build sdist + wheels
• Replacing unmaintained bumpversion with successor bump-my-version

If you want to sanity check the new build:

$ uv build -o ./dist/before
$ git switch pkg_resources && rm *.egg-info && uv build -o ./dist/after
$ uvx diffoscope --exclude-directory-metadata=recursive ./dist/before/*.tar.gz ./dist/after/*.tar.gz