BUG: Series[int] + datetime64 should raise
identified [here](https://github.com/pandas-dev/pandas/commit/a6c0d02c99cfaf90119d1316dd897019326472c1#r34783276).  We handle this correctly for timedelta64, but apparently not datetime64

```
iser = pd.Series(range(3))
dt = np.datetime64(3, "D")

>>> iser + dt
0   1970-01-04
1   1970-01-05
2   1970-01-06
```

Should behave like `iser + pd.Timestamp(dt)`:

```
>>> iser + pd.Timestamp(dt)
[...]
ValueError: Cannot add integral value to Timestamp without freq.
```