Fix for crash when deleting via admin site

[matthewhegarty]

Problem

I have verified that there is a crash if one tries to delete records using the admin site (see #432).

Solution

The fix is simple - to add instance attributes to the RowResult.

This introduced some code duplication, so I added a minor refactor of RowResult to set attributes via a new method.

• I also fixed sort order for conf.py attributes (raised by make lint).
• I added the pytz dependency which is needed for testing with python 3.9 (see Added python3.10 to tox environment list #1336).
• Added test file books-for-delete.csv

Acceptance Criteria

• Integration test to prove the issue exists and is fixed (test_admin_integration.py)
• Added unit tests for the new RowResult method

[coveralls]

Coverage increased (+0.009%) to 98.072% when pulling 0c3ab1e on matthewhegarty:error-on-delete-from-admin-432 into 19aa03d on django-import-export:main.

[manelclos]

LGTM