Refactor the implementations of tzinfo classes

[Aymeric Augustin]

Since the merge of the time zone support branch, django.utils.tzinfo and django.utils.timezone provide subtly different implementations of a tzinfo subclass representing local time.

The reason is explained at the bottom of ​this message. In short, the former is used for display and focuses on robustness; the latter is used for time zone support and focuses on correctness.

I think django.utils.tzinfo should be refactored and merged into django.utils.timezone, but that's probably not doable in a totally backwards-compatible fashion.

[Aymeric Augustin]

See also #18766.

[Aymeric Augustin]

Pull request: ​https://github.com/django/django/pull/1601

Discussion: ​https://groups.google.com/d/msg/django-developers/zwQju7hbG78/jxpGZfdlxlcJ

I'll merge this in a few days, unless the discussion on django-developers reveals some good reasons not to.

[Aymeric Augustin <aymeric.augustin@…>]

In ec2778b445546f624d3b3a1f2118e751b10bb2e7:

Fixed #17262 -- Refactored tzinfo implementations.

This commit deprecates django.utils.tzinfo in favor of the more recent
django.utils.timezone which was introduced when Django gained support
for time zones.

[Aymeric Augustin <aymeric.augustin@…>]

In d9413d33b2a8371731a92289123683cf6f440290:

Refactored code and tests that relied on django.utils.tzinfo.

Refs #17262.

[Tim Graham <timograham@…>]

In 1b0365ad34151f266fe3faecb4edf31a51a8642c:

Removed django.utils.tzinfo per deprecation timeline; refs #17262.