string-representation for Timestamp does not show nanoseconds if present
See:

```
In [27]: str(pd.Timestamp(200))
Out[27]: '1970-01-01 00:00:00'

In [28]: repr(pd.Timestamp(200))
Out[28]: "Timestamp('1970-01-01 00:00:00.000000200', tz=None)"
```

whereas microseconds work fine:

```
In [30]: str(pd.Timestamp(2000))
Out[30]: '1970-01-01 00:00:00.000002'
```
