Cannot convert a datetimelike to 'str' with astype
I find this surprising:

```
import pandas as pd
pd.Series([pd.Timestamp('2010-01-04 00:00:00')]).astype(str)
TypeError: cannot astype a datetimelike from [datetime64[ns]] to [|S0]
```

str(pd.Timestamp) works though, so why astype shouldn't?
Also the type representation "|S0" is not entirely clear to me.
