FIX: remove legacy API

[sabonerune]

Remove legacy APIs from __init__.py.

• Replace pkg_resources with importlib-resources.
  ref: https://importlib-resources.readthedocs.io/en/latest/migration.html
  This removes the implicit setuptools dependency.
  I think it is important for Python3.12 because setuptools may not be installed by default.
  ref: https://docs.python.org/3/whatsnew/3.12.html

gh-95299: Do not pre-install setuptools in virtual environments created with venv. This means that distutils, setuptools, pkg_resources, and easy_install will no longer available by default; to access these run pip install setuptools in the activated virtual environment.

• Remove six.

• Replace urlretrieve with urlopen.
  There is no name to give desc since it uses a TemporaryFile().