Modernize repo

[leplatrem]

• Replaced setup.cfg and setup.py by pyproject.toml
• Replaced flake8 by ruff
• Use requirements.in instead of list in setup.py
• Replaced manually created requirements.txt by pip-compile
• Remove dev-requirements.txt file and use extra dependencies instead
• Use version from git tag (setuptools-scm)
• Publish to Pypi on git tag
• Add config to categorize changes in autogenerated changelog
• Update release instructions
• Rename master to main
• Replace tox with GH actions
• Move tests out of package
• Move package to src

Things lost with this PR:

• Coverage badge / coveralls reporting
• No more changelog (now on GH releases)
• Description on Pypi does not contain changelog and contributors file contents anymore

[leplatrem]

       Copying src/kinto_attachment.egg-info to build/bdist.linux-x86_64/wheel/kinto_attachment-0.0.0-py3.11.egg-info
      running install_scripts
      error: [Errno 2] No such file or directory: 'build/bdist.linux-x86_64/wheel/kinto_attachment-0.0.0-py3.11.egg-info'
      [end of output]
  

[grahamalama]

(pushed to the wrong branch, sorry for the noise)