Removed tests for django.utils.unittest vs. unittest.

Silenced warnings caused by the deprecation of django.utils.unittest.

Thanks Preston Timmons and Carl Meyer for their advice.

Fixed #20680.
This doesn't need to be a package any more.

Refs #20680.
Fixed a few more imports of django.utils.unittest.

One import per line please! Refs #20680.
Deprecated django.utils.dictconfig.

The module still exists for whoever uses it in its current state, but it
isn't imported by Django anywhere.
Reflected the new default value of LOGGING_CONFIG in docs.
Relied on NullHandler from the logging stdlib module.
Fixed #20429 -- Added QuerySet.update_or_create

Thanks tunixman for the suggestion.