Modernize packaging

[medmunds]

Switch to pyproject.toml packaging, using hatchling.

• Replace all uses of setup.py with updated equivalent.
• Remove AUTHORS.txt (it just referred to GitHub).
• Move separate requirements for dev, docs, tests into their own requirements.txt files.
• Update testing documentation
• Update docs readme rendering to match PyPI (and avoid setup.py)
• Use hatch custom build hook to freeze version number in readme (previously custom setup.py code)
• In tox tests, use isolated builds and update pip