Remove setup.cfg

[FantasqueX]

What is the problem that this PR addresses?

Since questionary uses poetry instead of setuptools, it's better not keep setup.cfg. Mypy can be configured using pyproject.toml.
How did you solve it?

https://mypy.readthedocs.io/en/stable/config_file.html

Checklist

• I have read the Contributor's Guide.
• I will check that all automated PR checks pass before the PR gets reviewed.

[kiancross]

Great, thanks!