Correctly format missing Pillow/PIL exceptions messages. refs #19934
Avoid importing the deprecated `django.utils.importlib` package.
Fixed an aggregation test failure on MySQL.
Fixed #20943 -- Weakly reference senders when caching their associated receivers
Fixed #14043 -- Made sure nullable o2o delete works as expected

There was an old complaint about nullable one-to-one field cascading
even when the o2o field was saved to None value before the deletion.
Added an test to verify this doesn't happen.

Also some PEP 8 cleanup.
Fixed #14056 -- Made sure LEFT JOIN aren't trimmed in ORDER BY

If LEFT JOINs are required for correct results, then trimming the join
can lead to incorrect results. Consider case:

TBL A: ID | TBL B: ID  A_ID
       1           1   1
       2
Now A.order_by('b__a') did use a join to B, and B's a_id column. This
was seen to contain the same value as A's id, and so the join was
trimmed. But this wasn't correct as the join is LEFT JOIN, and for row
A.id = 2 the B.a_id column is NULL.
Fixed test failure caused by different NULL ordering between backends
Fixed #20820 -- Model inheritance + m2m fixture loading regression

Tests by Tim Graham, report from jeroen.pulles@redslider.net.
Fixed invalid testing fixture
Oracle also treats NULLs as largests values when ordering.
Fixed #20933 -- Allowed loaddata to load fixtures from relative paths.
Fixed #18967 -- Don't base64-encode message/rfc822 attachments.

Thanks Michael Farrell for the report and his work on the fix.
Import test case classes from their public API module.
Documentation - Noted that OneToOneField doesn't respect unique.

Added OneToOneField to the list of model fields for which the unique
argument isn't valid. (OneToOneFields are inherently unique, and if
the user supplies a value for unique it is ignored / overwritten.)
Fixed #20946 -- model inheritance + m2m failure

Cleaned up the internal implementation of m2m fields by removing
related.py _get_fk_val(). The _get_fk_val() was doing the wrong thing
if asked for the foreign key value on foreign key to parent model's
primary key when child model had different primary key field.
Fixed #12422 -- Don't override global email charset behavior for utf-8.

Thanks simonb for the report, Claude Paroz and Susan Tan for their work
on a fix.
Switched mail tests to SimpleTestCase.
Fixed a regression with get_or_create and virtual fields.

refs #20429

Thanks Simon Charette for the report and review.
Fixed docstring typo, thanks minddust.
Added myself to the committers list.