BUG: DatetimeIndex.get_loc failing when key is int (introduced in v1.1.0)
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- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

In version 1.0.5 the below was running successfully, but in 1.1.0, this is now broken as `int` keys are not accepted anymore (a `KeyError` is raised because the type of the key is now unexpected).

```
import numpy as np
from numpy import datetime64
from pandas import DatetimeIndex

dt = datetime64("2020-08-12T00:00:00.000000000")
dt_item = np.array(dt).item()
index = DatetimeIndex(data=[dt])
print(index.get_loc(dt))
print(index.get_loc(dt_item))
```