timedelta is considered as np.number?
When querying numbers from dataframe with `df.select_dtypes`, if i choose `include=np.number`, then timedelta will be also selected. But based on documentation, this should not happen. I checked the tests, it seems allow this to happen on purpose, then how should I only select numeric columns? should DOC be changed at least?

```python
df = pd.DataFrame({'a': pd.date_range("2012-01-01", periods=2), 
                   'b': np.random.randn(2), 
                   'd': pd.date_range("2012-01-01", periods=2).astype(str),
                   'e': pd.date_range("2012-01-01", periods=2, tz='UTC'),
                   'f': pd.timedelta_range("1 days", periods=2)
})
df.select_dtypes(include=['number'])
```
If diving to code, it seems because `timedelta` also is represented by `np.timedelta64` and is subclass of `np.number`.

Output:
the column b, f are selected

Expected Output:
only column b will be selected as a dataframe. Or a doc change instead?
