chore: Add project classifiers in pyproject.toml

[FBruzzesi]

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

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It's not much

[FBruzzesi]

We are not really testing anything in a free threading way. We mostly rely on underlying libraries to be compliant, but we don't have test for anything narwhals introduces

[MarcoGorelli]

thanks - ok with the license, but i don't think we need to list all the python versions, in all likelihood any version of narwhals will be forwards-compatible with new versions of python

[FBruzzesi]

thanks - ok with the license, but i don't think we need to list all the python versions, in all likelihood any version of narwhals will be forwards-compatible with new versions of python

I think it's nice to have, especially for versions that reached EOL. What should be assumed for a generic "Programming Language :: Python :: 3"? For instance, we still support 3.9 which is EOL, but in some time that will be no longer the case. It's very low effort to keep these in sync.