Add compatibility with Django 3.1

Author: askoretskiy

psqlextra/backend/migrations/state/model.py

@@ -1,3 +1,4 @@
+from collections.abc import Mapping
 from typing import Type
 
 from django.db.migrations.state import ModelState
@@ -68,7 +69,15 @@ def render(self, apps):
                 "Cannot resolve one or more bases from %r" % (self.bases,)
             )
 
-        fields = {name: field.clone() for name, field in self.fields}
+        if isinstance(self.fields, Mapping):
+            # In Django 3.1 `self.fields` became a `dict`
+            fields = {
+                name: field.clone() for name, field in self.fields.items()
+            }
+        else:
+            # In Django < 3.1 `self.fields` is a list of (name, field) tuples
+            fields = {name: field.clone() for name, field in self.fields}
+
         meta = type(
             "Meta",
             (),