Found cases where testing code was doing

    try:
        whatever
    except (some excption type):
        self.fail("exception shouldn't be thrown")

replaced it with just

    whatever

as this makes the unexpected errors easier to debug, and the tests
would fail just as much and aren't rendered less readable.

Thanks Markus Holtermann for review

Thanks Steve Kossouho for the report and original patch.

Dropped the DJANGO_TEST_TEMP_DIR environment variable.

Before this change, proper removal depended on the developer passing
dir=os.environ['DJANGO_TEST_TMP_DIR'] to tempfile functions.

The following are recognized as ending a line: the Unix end-of-line
convention '\n', the Windows convention '\r\n', and the old
Macintosh convention '\r'.

http://www.python.org/dev/peps/pep-0278

Thanks tchaumeny for review.

Added condition to prevent checking the existence of a file name of a
file like object when the name attribute is None. This is necessary
because a SpooledTemporaryFile won't exist on the file system or have a
name until it has reached its max_size. Also added tests.

refs #19934.

Due to a mixup between text and bytes, iteration over
a File instance was broken under Python 3.

Thanks to trac user pdewacht for the report and patch.

Added comments in the three empty models.py files that are still needed.

Adjusted the test runner to add applications corresponding to test
labels to INSTALLED_APPS even when they don't have a models module.

Just moving around some tests to be more logically grouped.

Avoids failures on Windows. Refs #8918.

Thanks leahculver for the suggestion and dc and vajrasky for work
on the patch.

- Fixed test that didn't close the files it opened
- Caught and handled exception when temp directory cannot be removed

- TemporaryFile now minimally mocks the API of the Python standard
  library class tempfile.NamedTemporaryFile to avoid AttributeError
  exceptions.
- The symbol django.core.files.NamedTemporaryFile is actually assigned
  as a different class on different operating systems.
- The bug only occurred if Django is running on Windows, hence why it
  was hard to diagnose.

Should be unneeded with Python 2.7 and up.
Added some unicode_literals along the way.

Refs #20680.

Thanks to kux for the report, and Russ Webber for the patch.