Django-select2

* Fix 52 -- Get all dependent field values
* Fix distinct search results if needed
* Add Django 3.2 to CI suite

* Add CSS support for Django admin
* Add support for dependent m2m fields

* Fix 39 -- Make default search more like Django admin 43
Partially revert 07054b2d8ff15144c6b1b00577f3c898d47712bc

* Fix 29 -- Do not filter based on empty search term

Split search terms only for __contains queries

Split search terms only for `__contains` queries and not for
`__startswith` or `__endswith`. We no also split not only be
whitespace but tab and newline.

If multiple search fields are defined conditions combined with
and OR. If single word matches in a for contains queries are
OR combined as well.

* Add official Django 3.1 support
* Add official Python 3.9 support
* Drop official Django 3.0 support
* Drop official Python 3.6 support
* Drop official Python 3.7 support
* Various documentation improvements