Allow imports from wagtail_modeladmin

[soh55]

When using this library with wagtail_modeladmin == 2.0.0 we face an ImportError from wagtail_hooks.py. This is caused by the following imports:

from wagtail.contrib.modeladmin.helpers import ButtonHelper, PermissionHelper
from wagtail.contrib.modeladmin.options import ModelAdmin, modeladmin_register
from wagtail.contrib.modeladmin.views import DeleteView, IndexView

I've added a try-except to import first from wagtail_modeladmin, then fall back to the original namespace.

[marteinn]

Hi @soh55 and thanks for the PR! I have reviewed it and since modeladmin is deprecated in Wagtail 5 we can just bundle wagtail-modeladmin. So I went with this approach. You can see my approach here ae51631

I'm closing this PR as we now have Wagtail 6 in wagtail-trash 3.0.0, but many thanks for letting us know that the plugin did not support Wagtail 6! 🙏