3.0.1 switched to a deprecated `importlib.metadata` API that's removed in Python 3.12

The 3.0.1 version switches from `pkg_resources` to `importlib.metadata`. However, it uses the deprecate pre-3.10 API that doesn't work with Python 3.12 anymore:

```pytb
$ tox -e py312
.pkg: _optional_hooks> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
.pkg: get_requires_for_build_sdist> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
.pkg: get_requires_for_build_wheel> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
.pkg: prepare_metadata_for_build_wheel> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
.pkg: build_sdist> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
py312: install_package> python -I -m pip install --force-reinstall --no-deps /tmp/pygal/.tox/.tmp/package/5/pygal-3.0.1.tar.gz
py312: commands[0]> coverage run --source=pygal /tmp/pygal/.tox/py312/bin/py.test pygal/test --junitxml=junit-py312.xml --flake8
ImportError while loading conftest '/tmp/pygal/pygal/test/conftest.py'.
pygal/__init__.py:64: in
for entry in entry_points().get('pygal.maps', ()):
E AttributeError: 'EntryPoints' object has no attribute 'get'
py312: exit 4 (0.93 seconds) /tmp/pygal> coverage run --source=pygal /tmp/pygal/.tox/py312/bin/py.test pygal/test --junitxml=junit-py312.xml --flake8 pid=149296
.pkg: _exit> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
py312: FAIL code 4 (7.79=setup[6.86]+cmd[0.93] seconds)
evaluation failed :( (7.91 seconds)
```

See "compatibility note" at the bottom of:
https://docs.python.org/3.11/library/importlib.metadata.html#entry-points

> For compatibility, if no parameters are passed to entry_points, a `SelectableGroups` object is returned, implementing that dict interface. In the future, calling `entry_points` with no parameters will return an `EntryPoints` object. Users should rely on the selection interface to retrieve entry points by group.

This is what happens in 3.12, so `.get()` is no longer valid.

Start in pygal/__init__.py, where entry_points() is used while loading map plugins, and review the compatibility note for importlib.metadata. Run tox -e py312 and the tests under pygal/test first. Done means the Python 3.12 test run no longer raises the EntryPoints.get() error and the existing tests pass.

Written by the indexing model from the issue text.