Add a hypothesis test for __getitem__

[sjdenny]

Compares polars __getitem__ calls with pandas & pyarrow.

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

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

Related issues

• Related issue #
• Closes test: add hypothesis test for DataFrame.__getitem__ #1008

Checklist

• Code follows style guide (ruff)
• Tests added
• Documented the changes

If you have comments or can explain your changes, please do so below.

Putting this up for initial feedback - there's a bunch of cases which pyarrow doesn't support (documented in the test). Do we want to support these cases, or should be instead tighten the test (on pandas as well) and declare these unsupported for now?

Cases:

• pyarrow: doesn't support negative indexes
• pyarrow: pairs of slices unsupported, e.g. df[0:1, 'a':'b'] (trivial pairs such as df[:, 'a':'b'] where one slice is : are an exception).
• pyarrow: empty edge case, e.g. df[[], "a":] fails.
• pyarrow & pandas: df[..., ::step] is unsupported, ie slice(None, None, <something>)

[MarcoGorelli]

this is awesome, thanks @sjdenny !

looks like there's a failure in the "random versions" CI job (specifically, pandas==2.0.3)

if we can address that, then I think we can merge this can then gradually address the rest