np.str_ raises TypeError: Argument must be a string

Passing a np.str_ to whenever.Time.parse_common_iso (and, I guess, other function expecting a `str` as well) raises an error :

```python
x = np.str_('00:00:00')

whenever.Time.parse_common_iso(x)
> TypeError: Argument must be a string
```
However :
```python
whenever.Time.parse_common_iso(str(x))
> Time(00:00:00)
```
