Modernize packaging setup

[zahlman]

These changes allow the project to go forward following modern packaging standards, entirely dropping dependency on running arbitrary code from setup.py (which is normally not required for pure Python projects).

• Instead of trying to implement a post-install hook (which wouldn't ordinarily run nowadays anyway), the logic for updating old config files is pushed into the startup check for a missing config file.

• Project metadata is specified using pyproject.toml, following the standards described in:

  • https://packaging.python.org/en/latest/specifications/pyproject-toml/
  • https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#writing-pyproject-toml

• As a result, the sdist includes a PKG-INFO metadata file that could in principle be used by other tooling to read project metadata, without needing to build the project (from their perspective: slow and insecure).

• With these changes, using the new functionality, a separate MANIFEST.in file is also no longer necessary. All project configuration is done declaratively, in one place.

[zahlman]

Please note that support for setuptools.command.* modules is liable to disappear over time, since running setup.py manually is deprecated - Setuptools is now only supposed to be used as a "build backend" (i.e., called by other tools to make sdists and wheels, not used directly as any kind of project manager). Recently, setuptools.command.test was temporarily removed causing a large amount of disruption, in many cases even to packages that didn't actually have any use for the functionality. As a general rule, dependency on setup.py should be eliminated or minimized whenever possible. (The advantages of static metadata in source distributions are clear to anyone who has experienced modern packaging in other languages or environments such as Node, Rust or Go.)