Add support for Django 3.1

.travis.yml

@@ -27,6 +27,11 @@ matrix:
     - { env: TOXENV=py37-django30, python: 3.7 }
     - { env: TOXENV=py38-django30, python: 3.8 }
     - { env: TOXENV=py38-django30-postgres DB=postgres, python: 3.8 }
+    # Django 3.1: Python 3.6, 3.7 or 3.8
+    - { env: TOXENV=py36-django31, python: 3.6 }
+    - { env: TOXENV=py37-django31, python: 3.7 }
+    - { env: TOXENV=py38-django31, python: 3.8 }
+    - { env: TOXENV=py38-django31-postgres DB=postgres, python: 3.8 }
     # Django development master (direct from GitHub source):
     - { env: TOXENV=py36-djangomaster, python: 3.6 }
     - { env: TOXENV=py37-djangomaster, python: 3.7 }

polymorphic/query.py

@@ -6,7 +6,7 @@ import copy
 from collections import defaultdict
 
 from django.contrib.contenttypes.models import ContentType
-from django.db.models import FieldDoesNotExist
+from django.core.exceptions import FieldDoesNotExist
 from django.db.models.query import ModelIterable, Q, QuerySet
 
 from . import compat

polymorphic/query_translate.py

@@ -7,7 +7,7 @@ from collections import deque
 
 from django.apps import apps
 from django.contrib.contenttypes.models import ContentType
-from django.core.exceptions import FieldError
+from django.core.exceptions import FieldError, FieldDoesNotExist
 from django.db import models
 from django.db.models import Q
 from django.db.models.fields.related import ForeignObjectRel, RelatedField
@@ -187,7 +187,7 @@ def translate_polymorphic_field_path(queryset_model, field_path):
                 # Can also test whether the field exists in the related object to avoid ambiguity between
                 # class names and field names, but that never happens when your class names are in CamelCase.
                 return field_path  # No exception raised, field does exist.
-        except models.FieldDoesNotExist:
+        except FieldDoesNotExist:
             pass
 
         submodels = _get_all_sub_models(queryset_model)

setup.cfg

@@ -17,6 +17,7 @@ classifiers =
 	Framework :: Django :: 2.1
 	Framework :: Django :: 2.2
 	Framework :: Django :: 3.0
+	Framework :: Django :: 3.1
 	Intended Audience :: Developers
 	License :: OSI Approved :: BSD License
 	Operating System :: OS Independent

tox.ini

@@ -1,9 +1,9 @@
 [tox]
 envlist =
 	py35-django{21,22,master}
-	py36-django{21,22,30,master}
-	py37-django{21,22,30,master}
-	py38-django{21,22,30,master}
+	py36-django{21,22,30,31,master}
+	py37-django{21,22,30,31,master}
+	py38-django{21,22,30,31,master}
 	docs
 
 [testenv]
@@ -17,6 +17,7 @@ deps =
 	django21: Django ~= 2.1
 	django22: Django ~= 2.2
 	django30: Django ~= 3.0
+	django31: Django ~= 3.1
 	djangomaster: https://github.com/django/django/archive/master.tar.gz
 	postgres: psycopg2
 commands =