Switch to pyproject.toml based project setup

[Daverball]

Based on the equivalent changes to morepath made by @henri-hulski .

I did however decide to add flake8-pyproject to the development dependencies, so we can put the flake8 configuration inside pyproject.toml as well.

Long term it might make more sense to replace black and flake8 with ruff.

I also decided to use tox-gh instead of creating a separate runner for every tox environment, since that seems a bit excessive, considering how fast most checks are.

This is preparation for adding inline type hints to reg, I wanted a more modern project configuration, before I start adding type checkers to it.

[henri-hulski]

Seems to be a problem with coverall. I think we should remove it as we have 100% coverage.

If you want to switch to rust feel free to go. I'm also starting to switch my projects. I quite like it.

[Daverball]

Seems to be a problem with coverall. I think we should remove it as we have 100% coverage.

I think it was just a temporary network failure, the same job succeeded on the forked repository. Feel free to re-run the failed jobs.

If you want to switch to rust feel free to go. I'm also starting to switch my projects. I quite like it.

I'm happy to do that in a follow-up PR, for now this is good enough.