peewee+MySQL TimeField returns timedelta object
The fix does not work. The `elif` branch is never reached because the timedelta object evaluates to `True`, so the behavior is the same as before.

When using MySQL, the `value` that gets passed into `TimeField.python_value(value)` is a `timedelta` object, which doesn't get properly handled (to turn it into a `time` object).

Modifying `TimeField.python_value` to the following fixes the issue (note the `is None`, as `time(0)` (midnight) evaluates to `False`):

``` python
class TimeField(_BaseFormattedField):
    ...
    def python_value(self, value):
        if value and isinstance(value, basestring):
            pp = lambda x: x.time()
            return format_date_time(value, self.formats, pp)
        elif value and isinstance(value, datetime.datetime):
            return value.time()
        elif value is not None and isinstance(value, datetime.timedelta):
            return (datetime.datetime.min + value).time()
        else:
            return value
```
