Allow "None" for conf.TIME_ZONE to avoid calling tzset()

[philz42]

Django defaults (I believe) to Chicago time. It often makes sense to set the time zone explicitly, but, in many applications, the system timezone (typically handled by /etc/localtime magic) is just fine. I'd like to propose that

if hasattr(time, 'tzset'):

# Move the time zone info into os.environ. See ticket #2315 for why
# we don't do this unconditionally (breaks Windows).
os.environTZ = self.TIME_ZONE
time.tzset()

be changed to start "if self.TIME_ZONE is not None and hasattr(time, 'tzset'): ...".

That way, if you explicitly set TIME_ZONE to None (this could be the default or not; doesn't matter to me), the system default will get used.

Thoughts?