update Django to compatibility with 3.1.3 based on passing tests (#58)

.travis.yml

@@ -7,6 +7,8 @@ python:
   - "3.5"
   - "2.7"
 env:
+  - DJANGO_VERSION=">=3.1.0,<3.2"
+  - DJANGO_VERSION=">=3.0.0,<3.1"
   - DJANGO_VERSION=">=2.2.0,<2.3"
   - DJANGO_VERSION=">=2.1.0,<2.2"
   - DJANGO_VERSION=">=2.0.0,<2.1"
@@ -27,6 +29,14 @@ jobs:
       env: DJANGO_VERSION=">=2.1.0,<2.2"
     - python: "2.7"
       env: DJANGO_VERSION=">=2.0.0,<2.1"
+    - python: "2.7"
+      env: DJANGO_VERSION=">=3.0.0,<3.1"
+    - python: "2.7"
+      env: DJANGO_VERSION=">=3.1.0,<3.2"
+    - python: "3.5"
+      env: DJANGO_VERSION=">=3.0.0,<3.1"
+    - python: "3.5"
+      env: DJANGO_VERSION=">=3.1.0,<3.2"
   include:
     - stage: lint
       install:

dev-requirements.txt

 -r dev-env-requirements.txt
-django==2.2.13
+django==3.1.4

setup.py

@@ -41,5 +41,7 @@ setup(
         'Framework :: Django :: 2.0',
         'Framework :: Django :: 2.1',
         'Framework :: Django :: 2.2',
+        'Framework :: Django :: 3.0',
+        'Framework :: Django :: 3.1',
     ],
 )