COMPAT: Argsort position matches NumPy
Closes https://github.com/pandas-dev/pandas/issues/29884

Bit of a WIP, CI is probably going to fail here. One API question, do we want NaT to sort at the end, regardless of the NumPy version? Or should we match the behavior of the installed NumPy (NaT at the start for <1.18, at the end for >=1.18)?