Add importlib_metadata as a real dependency for now

[akx]

This is an alternate take to #546 that will make this package installable in general without having to specify importlib_metadata as a downstream dependency.

Using importlib.metadata directly (as implemented in #546) is the better solution, but this should allow for releasing a 3.0.4 as a stopgap measure.

[akx]

cc @gentooboontoo, if you want to release a 3.0.4 today before I get CI up and running and get a better fix in.

I locally tested that this builds a correct wheel that has the correct dependency:

$ unzip -p dist/pygal-3.0.3-py2.py3-none-any.whl pygal-3.0.3.dist-info/METADATA | grep -C5 importli
Classifier: Topic :: Multimedia :: Graphics :: Presentation
Provides: pygal
Requires-Python: >=3.8
Description-Content-Type: text/x-rst
License-File: COPYING
Requires-Dist: importlib-metadata
Provides-Extra: docs
Requires-Dist: sphinx ; extra == 'docs'
Requires-Dist: sphinx-rtd-theme ; extra == 'docs'
Requires-Dist: pygal-sphinx-directives ; extra == 'docs'
Provides-Extra: lxml

[gentooboontoo]

Thanks. I have already added install_requires in 0a79838 but without a comment and I also forgot to remove importlib_metadata from tox.ini so PR rebased and merged 😉