chore(build): switch to Poetry

[stegayet]

Follow-up on celery/celery#8238.

This PR aims at switching from setup.py, setup.cfg, MANIFEST.in files to PEP-517 compliant pyproject.toml using poetry dependency management and poetry-core build backend.

Global considerations

I aimed at being iso with current dependencies and wheel content. I have some improvements in mind that will be done in follow-up PRs.

Dependencies management

• Replace requirements* files with single pyproject.toml to handle all dependencies.
• I encountered an issue with latest version of flake8 not being compatible with Python 3.7. IMO this is not an issue as Python 3,7's EOL is June 2023, so we might want to drop support this specific version.

Global pyproject.toml configuration

TODO

Test strategy

• CI is green
• Proceed to local installation
• Check wheel content
• Check dependency constraints
• Upload to https://test.pypi.org/ and check information

TODO

• Update documentation