BUG: isin - dtype conversions (Timestamp vs ints)
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

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

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related: #34125 : could be closed in favor of that if it is too much of a duplicate.

#### Code Sample, a copy-pastable example

```python
import pandas as pd
s = pd.Series(pd.date_range("jan-01-2013", "jan-05-2013"))
s.isin([1356998400000000000])
>>>
0     True
1    False
2    False
3    False
4    False
dtype: bool
```

#### Problem description

Problem arises here because `algos._ensure_dtype(s)` returns an `np.int64` array (that matches the data) and `dtype=<M8[ns]`. 

#### Expected Output

```py
0    False
1    False
2    False
3    False
4    False
dtype: bool
```

