allow importing of Wagtail modeladmin from contrib or external

Wagtail 6.0 removed the deprecated wagtail.contrib.modeladmin app. It is available as an external package https://github.com/wagtail-nest/wagtail-modeladmin

wagtailsvg/wagtail_hooks.py

@@ -1,7 +1,13 @@
-from wagtail.contrib.modeladmin.options import (
-    ModelAdmin,
-    modeladmin_register
-)
+try:
+    from wagtail.contrib.modeladmin.options import (
+        ModelAdmin,
+        modeladmin_register
+    )
+except ImportError:
+    from wagtail_modeladmin.options import (
+        ModelAdmin,
+        modeladmin_register
+    )
 
 try:
     from wagtail import hooks