datetime.datetime.astimezone should take the more general tzinfo instead of its subclass timezone

`timezone` inherits from `tzinfo`, so using `tzinfo` is more general (and e.g. pytz types inherit directly from `tzinfo`).

This is the correct type, to quote `astimezone`'s code:

``` py
elif not isinstance(tz, tzinfo):
    raise TypeError("tz argument must be an instance of tzinfo")
```
