Move project metadata to pyproject.toml, clean-up

[mwtoews]

There are perhaps too many changes here, but here's a summary of the main items:

• Specify a package build backend, based on setuptools (PEP 517 and PEP 518)
• Move most of the static project metadata from setup.py to pyproject.toml (PEP 621), as supported by setuptools

Transferring metadata from setup.py to pyproject.toml is nearly 1:1 with a few changes:

• Dependencies are moved from requirements/production.txt to the dependencies section. These are now installed via pip install ., along with the package.
• Three optional dependency groups (or "extras") are created: lint, test and dev, where dev includes both lint and test. These can be installed (e.g.) pip install .[lint]. Perhaps others should be created, e.g. "doc"?
• Home and Repository URLs are updated
• Readme content is adjusted with an updated URL to Doxygen and hopefully fixed RST rendition (currently it has a line-break)
• Platforms is a setuptools-specific metadata item, and is not used by pip, so it is removed

Other changes:

• Remove setup.cfg, which seems to not be needed, except for .flake8 configuration
• The package version is dynamically read from breathe.__init__.__version__, so version-check is unnecessary
• Use python3 -m build (docs) instead of directly invoking setup.py
• "# -- coding: utf-8 --" is unnecessary, since the default encoding for Python 3 files is utf-8
• Add "furo" to development dependency, as this seemed to be missing.

[michaeljones]

Thank you for the contribution. I'll try to check it out this week. Certainly looks like good work.

I've only come across pyproject.toml files when using poetry but my understanding is that they have been the standard for Python projects for a bit now?

[mwtoews]

Yes, pyproject.toml is here to stay as a configuration file for a wide range of Python projects.

[michaeljones]

Looks good to me. I'd rather wait until the next release is out to merge this. Unless @vermeeren would like to merge this and see how it impact the release process.

I'm not sure when @vermeeren might have time to attempt a release.