Fixed ticket #23998, datetime.time default value on migrations

When trying to use datetime.time(10) value for a django.db.models.fields.TimeField model default value on the default value questioner the system raises this exception:
```
The datetime module is available, so you can do e.g. datetime.date.today()
>>> datetime.time(10)
Traceback (most recent call last):
File "manage.py", line 10, in <module>
  execute_from_command_line(sys.argv)
[....]
File "/Users/tuxskar/.virtualenvs/caluny/lib/python2.7/site-packages/django/db/migrations/questioner.py", line 126, in ask_not_null_addition
    return eval(code, {}, {"datetime": datetime_safe})
  File "<string>", line 1, in <module>
TypeError: 'module' object is not callable
```
This patch fix it
I'm referring to the `time.strftime` method you added.