Fixed #20640 -- Avoided NoReverseMatch in get_deleted_objects

The default delete action resulted in a NoReverseMatch if it were to
list any Model with a ModelAdmin with `get_urls` overridden to remove
the change url.  Catching the error and not displaying the link in that
case, as was already done for models with no registered admins.

Thanks Keryn Knight for the report.
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 #5335 -- Changed ErrorDict to be a subclass of defaultdict.