chore: fully switch to project/packaging configuration to pyproject.toml

[stegayet]

As of now, kombu uses setup.cfg, setup.py, and MANIFEST.in as project/packaging configuration files besides pyproject.toml. The idea would be to fully switch to the latter.

Some of those configuration items only needs to be migrated to pyproject.toml (mypy for instance), for some others, it will require more work (flake8 does not support pyproject.toml and could be replaced with ruff).

Note that this issue does not cover kombu package manager/build backend modernization.

I can work on this issue.