Fix Parquet Serialization Error from pyarrow string to largestring

[wilsonbb]

In test_io.py we test a roundtrip serialization of a NestedFrame with a string-based "band" column.

During this, the pyarrow type gets converted from a string to large string. This is not a critical breakage, but we hacked the unit test dataframe to get around it in this commit. We should be able to revert that change and then properly fix the test.