REF: make maybe_cast_result_dtype a WrappedCythonOp method
Abstraction-wise, this is a much better place for it.
can you rebase rebased + greenish can you merge master (just because merge lots of groupby things) hah i was just about to comment "no merge conflict".  will do. rebase + greenish (just the ResourceWarning that apparently we _still_ arent suppressing and the pyarrow stuff thats failing everywhere)