re-add resource_class deprecation warning

[matthewhegarty]

Problem

Closes #1823

Re-added the deprecation warnings from v3.

Now if you declare:

class BookAdmin(ImportExportModelAdmin):
    list_display = ("name", "author", "added")
    list_filter = ["categories", "author"]
    resource_class = BookResource
    change_list_template = "core/admin/change_list.html"

You will see the following warning if you enable deprecation warnings, and browse to the import or export page in the UI:

python -X dev ./manage.py runserver

 DeprecationWarning: The 'resource_class' field has been deprecated. Please implement the new 'resource_classes' field

Acceptance Criteria

• Re-added tests

[coveralls]

coverage: 100.0%. remained the same
when pulling 8a4c710 on matthewhegarty:issue-1823-readd-resource-deprecation-warnings
into b2479b3 on django-import-export:main.