Using OrderedDict in GroupBy Module(s)
Spurred by conversation in https://github.com/pandas-dev/pandas/pull/25224#discussion_r264786749 right now we are using plain old dictionaries in most of the groupby aggregation functions. For Py35 this makes the order of the returned result non-deterministic.

We could theoretically make a stronger assertion around the order here by replacing those dicts with OrderedDicts