Clearer explanation when exception has no message

"No exception supplied" is misleading; actually there is an exception,
but there's no message string.
Don't hard-code class names when calling static methods

normalize_email should be called on the instance, not the class. This
has the same effect normally but is more helpful to subclassers. When
methods are called directly on the class, subclasses can't override
them.
Fixed regroup example.

Chicago was missing.
Made an app-cache test not dependent on all models in defer_regress
Fixed #16436 -- defer + annotate + select_related crash

Correctly calculate the ``aggregate_start`` offset from loaded fields,
if any are deferred, instead of ``self.query.select`` which includes all
fields on the model.

Also made some PEP 8 fixes.
Fixed #20509 - Proper parsing for dumpdata --pks option.

Thanks weipin for the report and Baptiste Mispelon for the patch.
Fixed #20272 - Moved update_fields existence check into Model._do_update.

Thanks Gavin Wahl.
Fixed #16856 - Added a way to clear select_related.

Thanks Carl for the suggestion and David Cramer for the patch.
Merge pull request #1027 from intgr/debug_no_exc_message

Clearer explanation when exception has no message
Fixed #19425 - Added InlineModelAdmin.get_extra hook.

Thanks dave@ for the suggestion and Rohan Jain for the patch.
Added missing stacklevel arg in forms/models.py
Fixed #20502 -- Flushed i18n caches for a defaultfilter test

Thanks Chris Wilson for the report and initial patch.
Tweak caching decorators/utility functions xrefs.
Fixed #16436 -- Oracle defer_regress test failure

Oracle doesn't like grouping by TextField, so use CharFields instead in
models.
Merge pull request #1232 from fusionbox/normalize_email

Fixed #20531 -- Don't hard-code class names when calling static methods
Fixed #20511 -- Corrected link about isolation levels in databases docs

Thanks tinodb for the report.
Fixed #20515 - Clarified note regarding translator comments.

Thanks wim@ for the suggestion.
Fixed #20326 - Corrected form wizard get_form() example.

Thanks tris@ for the report.
Fixed #20166 - Corrected TEST_DEPENDENCIES example in testing docs.

Thanks czambran.
Fixed #18681 -- BaseModelAdmin.get_form and InlineModelAdmin.get_formset no longer bypass get_fieldsets

Thanks msopacua for the report.