Remove remaining code from Django 3.1

Author: hramezani

.github/workflows/test.yml

@@ -21,8 +21,6 @@ jobs:
             django-version: '4.1'
           - python-version: '3.7'
             django-version: 'main'
-          - python-version: '3.10'
-            django-version: '3.1'
 
     services:
 

simple_history/tests/tests/test_models.py

@@ -1324,14 +1324,8 @@ def test_migrations_include_order(self):
 
         model_state = state.ModelState.from_model(SeriesWork.history.model)
         found = False
-        # `fields` is a dict in Django 3.1
-        fields = None
-        if isinstance(model_state.fields, dict):
-            fields = model_state.fields.items()
-        else:
-            fields = model_state.fields
-
-        for name, field in fields:
+
+        for name, field in model_state.fields.items():
             if name == "_order":
                 found = True
                 self.assertEqual(type(field), models.IntegerField)