Refactors Series.groupby to match DataFrame.groupby

[Daverball]

• Follow-up to Extends DataFrame.groupby overloads to recognize some scalar index types #679 (Related Issue DataFrame.groupby should return _DataFrameGroupByScalar when grouping by a PeriodIndex #674 has already been closed)
• Tests added: Please use assert_type() to assert the type of any return value

[Daverball]

I also lifted Index out of GroupByObjectNonScalar to avoid the unsafe overload errors and to get rid of one of the redundant overloads in DataFrame.groupby.

[Dr-Irv]

Thanks for the nice contribution. Can you try np.integer rather than using astype(np.int64) ?

[Dr-Irv]

Thanks @Daverball