Fix filter by EnumField

See merge request !8

src/documents/admin.py

 from django.contrib import admin
 from documents.models import Page, Document, DocumentLink, Section, Transcription
+from enumfields.admin import EnumFieldListFilter
 
 
 class PageAdmin(admin.ModelAdmin):
     list_display = ('id', 'name', 'page_type', 'nb', 'direction', )
-    list_filter = ('page_type', )
+    list_filter = [('page_type', EnumFieldListFilter)]
     fields = ('id', 'name', 'folio', 'page_type', 'nb', 'direction', 'image')
     readonly_fields = ('id', )
 
@@ -17,7 +18,7 @@ class DocumentLinkAdmin(admin.TabularInline):
 
 class DocumentAdmin(admin.ModelAdmin):
     list_display = ('id', 'name', 'type')
-    list_filter = ('type', )
+    list_filter = [('type', EnumFieldListFilter)]
     fields = ('id', 'type', 'name')
     readonly_fields = ('id', )
     inlines = [DocumentLinkAdmin, ]