TickTime can only format datetime64[ns], not other scales

```py
>>> int(np.datetime64("now", "ns"))
1531768357000000000
>>> int(np.datetime64("now", "us"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: int() argument must be a string, a bytes-like object or a number, not 'datetime.datetime'
```

This breaks TickTime's `tp_call()`, which can't convert such arguments.
