API: DatetimeIndex.get_loc(datetime) should require tzawareness compat?
```
dti = pd.date_range("2016-01-01", periods=3)
dti2 = dti.tz_localize("UTC").tz_convert("US/Pacific")

>>> dti.get_loc(dti2[0])
0
>>> dti2.get_loc(dti[0])
KeyError: Timestamp('2016-01-01 00:00:00-0800', tz='US/Pacific', freq='D')

ser = pd.Series(range(3), index=dti)
ser2 = pd.Series(range(3), index=dti2)

>>> ser.loc[dti2[1]]
1
>>> ser2.loc[dti[1]]
KeyError: Timestamp('2016-01-02 00:00:00-0800', tz='US/Pacific', freq='D')
```

get_loc is effectively an equality check, so I think it should behave like all our other comparisons and require tzawareness-compat.  So all of these would raise KeyError.

xref #17920, https://github.com/pandas-dev/pandas/pull/30819#issuecomment-572281006 