BUG: datetime64, timedelta64 comparisons do not return NotImplemented

I expected datetime64 and timedelta64 comparison methods to return NotImplemented when operating with pd.NaT, which has `__array_priority__ = 100`.


```
import numpy as np
import pandas as pd

ts = np.datetime64("now", "ns")
td = np.timedelta64(0, "ns")

assert pd.NaT.__array_priority__ == 100

>>> ts.__lt__(pd.NaT)
array(False)

>>> td.__lt__(pd.NaT)
array(False)
```