Introduce lower bound on attrs version

[alcarney]

lsprotocol uses the import attrs API which was only introduced in v21.3.0 .

This PR sets a minimum version for attrs in the python package's pyproject.toml to hopefully guard against issues like this one in the future.

Question: Is it sufficient to rely on the version bound set here to ensure the correct version is also installed for pygls and esbonio? Or since both downstream packages also import attrs themselves, is it more correct to have a version bound set in both pygls and esbonio? 🤔

[brettcannon]

Question: Is it sufficient to rely on the version bound set here to ensure the correct version is also installed for pygls and esbonio?

Yes, as your installer's resolver will have to make sure that any version of a distribution that gets installed is compatible with all requirements for that distribution. Basically pip has to walk the dependency graph and make sure whatever it chooses to install for attrs satisfies every requirement it comes across.

[alcarney]

Thanks!