Closed #23365 -- Makemigrations allows timezone-aware datetimes

When a user is adding a new, non-nullable datetime field to their model, 
they're now able to use django util's timezone module to supply a 
timezone-aware value. The prompt also shows an example using the 
preferred timezone.now() instead of datetime's naive now().
Thanks, I hope we can get away with a less repetitive solution. I pushed a couple updates in https://github.com/django/django/pull/3289, could you take a look?