Minor refactoring to support filtering exports

[matthewhegarty]

Problem

#1578
It is not possible to modify the Admin UI to include a custom field for export. This is possible for import but not export.

The bug is due to the fact that the form is removed from kwargs hence cannot be used in custom Resources to filter the queryset.

Solution

• changed the pop() call so that the form remains in kwargs
• Added a new filter_export() hook to make it a lot easier to dynamically filter querysets for export
• Updated the example app to demonstrate using 'E Book'

Acceptance Criteria

• Ran integration tests
• Manual testing
• Updated Documentation

Other refactoring

• Split the test fixtures into 'book', 'categories' and 'author'
• Split import / export tests in test_admin_integration.py for better readability

[coveralls]

Coverage: 100.0%. Remained the same when pulling fb6282f on matthewhegarty:issue-1578-filtering-export-from-form into 0ff664c on django-import-export:main.

[pokken-magic]

This looks like a useful tool. The general trend toward increasing granularity of operations makes sense to me and is something we will probably keep doing elsewhere.

[youtux]

Hi, can you please make a release with this fix? I can't customize the export form without this.

[matthewhegarty]

yes, my plan is to release it this week

[matthewhegarty]

Latest release (3.3.0) now available https://pypi.org/project/django-import-export/