Switch from_flat 'index' keyword to 'on'

[wilsonbb]

Switches the from_flat arguments to reflect the new API introduced in nested-pandas 3.0. We pin however to 3.1 to take advantage of the fix introduced in lincc-frameworks/nested-pandas#173

We also have a testing hot fix for a serialization error with a note to more fully address in #71

[github-actions]

Before [973feca]	After [49b29b3]	Ratio	Benchmark (Parameter)
509±5ms	596±9ms	1.17	benchmarks.NestedFrameQuery.time_run
250±3ms	257±2ms	1.02	benchmarks.NestedFrameReduce.time_run
141M	139M	0.98	benchmarks.NestedFrameAddNested.peakmem_run
248±5ms	243±2ms	0.98	benchmarks.NestedFrameAddNested.time_run
143M	141M	0.98	benchmarks.NestedFrameQuery.peakmem_run
140M	137M	0.98	benchmarks.NestedFrameReduce.peakmem_run

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