Make the admin base_model setting optional.

It can be detected using get_base_polymorphic_model()

example/pexp/admin.py

@@ -4,13 +4,13 @@ from pexp.models import *
 
 
 class ProjectAdmin(PolymorphicParentModelAdmin):
-    base_model = Project
+    base_model = Project  # Can be set explicitly.
     list_filter = (PolymorphicChildModelFilter,)
     child_models = (Project, ArtProject, ResearchProject)
 
 
 class ProjectChildAdmin(PolymorphicChildModelAdmin):
-    base_model = Project
+    base_model = Project  # Can be set explicitly.
 
     # On purpose, only have the shared fields here.
     # The fields of the derived model should still be displayed.
@@ -27,13 +27,12 @@ admin.site.register(ResearchProject, ProjectChildAdmin)
 
 
 class UUIDModelAAdmin(PolymorphicParentModelAdmin):
-    base_model = UUIDModelA
     list_filter = (PolymorphicChildModelFilter,)
     child_models = (UUIDModelA, UUIDModelB)
 
 
 class UUIDModelAChildAdmin(PolymorphicChildModelAdmin):
-    base_model = UUIDModelA
+    pass
 
 
 admin.site.register(UUIDModelA, UUIDModelAAdmin)
@@ -42,13 +41,12 @@ admin.site.register(UUIDModelC, UUIDModelAChildAdmin)
 
 
 class ProxyAdmin(PolymorphicParentModelAdmin):
-    base_model = ProxyBase
     list_filter = (PolymorphicChildModelFilter,)
     child_models = (ProxyA, ProxyB)
 
 
 class ProxyChildAdmin(PolymorphicChildModelAdmin):
-    base_model = ProxyBase
+    pass
 
 
 admin.site.register(ProxyBase, ProxyAdmin)