Summary:

Remove deprecated and outdated files.

Review Request #12004 — Created Jan. 26, 2022 and submitted Feb. 3, 2022, 10:26 p.m.

Description

This change removes a bunch of things which are deprecated or no longer
needed:

The datagrid and integrations apps had old templatetags files that
just raised warnings about their deprecation.
We had a number of backports of newer django functionality to allow us
to maintain compatibility with Django 1.6.
We had some stuff that provided compatibility with the importlib
module for Python versions that we no longer support (3.4 and below).
The old basictag decorator has been deprecated for a long time.

Testing Done

Ran unit tests.

Commits

Summary
	Remove deprecated and outdated files.

Screenshots

Issues

Description	From	Last Updated
This can be one statement now.	chipx86	Feb. 3, 2022, 5:17 p.m.
Should be in alphabetical order.	chipx86	Jan. 31, 2022, 3:58 p.m.
These were deleted as well.	chipx86	Jan. 31, 2022, 3:59 p.m.

flake8 passed.

JSHint passed.

Have we fully moved off of all these for Review Board?

Kinda want to hold off on this change until the end of this work.

I have a pending change to do that that I haven't yet posted.

I won't be landing any of this until we're ready to switch over on both branches.

djblets/testing/testcases.py (Diff revision 1)

Show all issues

This can be one statement now.

How is that? models_mod is used throughout this method.

Yeahhh I read that at the timeas return models_mod. Ignore me.

djblets/util/decorators.py (Diff revision 1)
Show all issues
```
Should be in alphabetical order.
```
docs/djblets/coderef/index.rst (Diff revision 1)
Show all issues
```
These were deleted as well.
```

Pushed to release-3.x (7708fff)