Declare dev dependencies using PEP 735 – Dependency Groups in pyproject.toml

[Avasam]

Unlike using [extras], PEP 735 – Dependency Groups in pyproject.toml allows declaratively configuring Project dependencies that won't end up exposed in the published package.

Here I have set a dev dependency group, which is the canonical group name to specify dev-dependencies for which certain package managers can take shortcuts (for example uv sync, uv pip install --dev, poetry sync, poetry install --dev, pipenv install --dev, etc. will install dev dependencies by default).

I have added 2 subgroups (checkers and type-checkers) for minimal dependency install on the CI. These group names match the CI job names.

This requires pip >=25.1 to use with pip: pip install --group=dev. Pip 25.1 dropped support for Python 3.8, but luckily since this is only used for checkers in CI, I don't need to wait for #2413 .