Switch to pyproject.toml (using hatchling)

[dhdaines]

Pull request

Switch to pyproject.toml. I used hatch or more specifically its build backend hatchling because it was convenient, but we could also use plain old setuptools if you prefer! Fixes #931

How Has This Been Tested?

hatch run build
hatch shell
pip install -e .[dev]
nox

Checklist

• [x ] I have read CONTRIBUTING.md.
• [x ] I have added a concise human-readable description of the change to CHANGELOG.md.
• [x ] I have tested that this fix is effective or that this feature works.
• [x ] I have added docstrings to newly created methods and classes.
• [x ] I have updated the README.md and the readthedocs documentation. Or verified that this is not necessary.

[dhdaines]

Note that the output sdist and wheels end up being named pdfminer_six-... and not pdfminer.six-.... This is probably a good thing but I don't know if it will cause problems for PyPI?

[dhdaines]

The test failure above is some random GitHub failure, not an actual problem with this PR.