Pandas 0.25.0 breaks np.isin
Observed with numpy 1.17.4 as well as (latest) 1.18.1:

```python
# pandas 0.24.2:
l = [pd.Timestamp]
pd.Timestamp == pd.Timestamp
>>> True
np.isin(l, l)
>>> array([ True])
```

```python
# pandas 0.25.0
l = [pd.Timestamp]
pd.Timestamp == pd.Timestamp
>>> True
np.isin(l, l)
>>> array([ False])
```


#### Problem description
Since 0.25.0, the `==` of `pd.Timestamp` is preserved but it doesn't go through `np.isin`.
This is observed as well under pandas 0.25.3.