Drop Django 3.1 support.

Author: hramezani

CHANGELOG.rst

@@ -6,6 +6,7 @@ Changelog
 
 * Add Python 3.10 support.
 * Add Django 4.0 support.
+* Drop Django 3.1 support.
 
 2.0.0 (2021-11-14)
 ~~~~~~~~~~~~~~~~~~

setup.cfg

@@ -12,7 +12,6 @@ classifiers =
     Environment :: Web Environment
     Framework :: Django
     Framework :: Django :: 2.2
-    Framework :: Django :: 3.1
     Framework :: Django :: 3.2
     Framework :: Django :: 4.0
     Intended Audience :: Developers

tox.ini

@@ -4,10 +4,9 @@ envlist =
     black
     flake8
     isort
-    py{36,37,38,39}-dj{22,31}
+    py{36,37,38,39}-dj22
     py{36,37,38,39,310}-dj32
-    py{38,39,310}-dj40
-    py{38,39,310}-djmain
+    py{38,39,310}-dj{40,main}
     docs
 
 [gh-actions]
@@ -21,7 +20,6 @@ python =
 [testenv]
 deps =
     dj22: Django>=2.2,<3.0
-    dj31: Django>=3.1,<3.2
     dj32: Django>=3.2,<3.3
     dj40: Django>=4.0,<4.1
     djmain: https://github.com/django/django/archive/main.tar.gz