to_timedelta returns an object of different type depending of Python version
Hello,

```
import pandas as pd
from datetime import timedelta
td=pd.to_timedelta('00:00:15')
```

Python 2

```
td
Timedelta('0 days 00:00:15')

isinstance(td, timedelta)
True
```

but with Python 3

```
td
numpy.timedelta64(15000000000,'ns')

isinstance(td, timedelta)
False
```

Kind regards
