Add 2 filters to get main type and subtype of media type. Jinja2 and Nunjucks don't handle
slices in the same way (Python way for Jinja2, JS way for Nunjucks), making it difficult
to retrieve main type of a media from media type. Thoses filters work in both cases.
- updated installation to use the now standard `pyproject.toml` instead of legacy
`setup.py`. `setup.py` and other legacy files have been deleted.
- removed outdated README4PACKAGERS
- removed pylintrc which has not been correctly reviewed. Linter conf should go to
`pyproject.toml` now.
- [hatch](https://hatch.pypa.io) is now used as main building tool. However, thanks to the
use of standards, other tools can be used too.
- update .hgignore
- several dependencies version bump, with code update to adapt to changes.
