Drop support for Django 3.1

Author: hramezani

setup.cfg

@@ -15,7 +15,6 @@ 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

@@ -2,14 +2,13 @@
 envlist =
     docs
     packaging
-    py{36,37}-dj{22,31,32}-{sqlite,postgresql,postgis,mysql}
-    py{38,39}-dj{22,31,32,40,main}-{sqlite,postgresql,postgis,mysql}
+    py{36,37}-dj{22,32}-{sqlite,postgresql,postgis,mysql}
+    py{38,39}-dj{22,32,40,main}-{sqlite,postgresql,postgis,mysql}
     py{310}-dj{32,40,main}-{sqlite,postgresql,postgis,mysql}
 
 [testenv]
 deps =
     dj22: django~=2.2.17
-    dj31: django~=3.1.3
     dj32: django~=3.2.9
     dj40: django~=4.0.0
     sqlite: mock
@@ -43,19 +42,19 @@ whitelist_externals = make
 pip_pre = True
 commands = make coverage TEST_ARGS='{posargs:tests}'
 
-[testenv:py{36,37,38,39,310}-dj{22,31,32,40,main}-postgresql]
+[testenv:py{36,37,38,39,310}-dj{22,32,40,main}-postgresql]
 setenv =
     {[testenv]setenv}
     DB_BACKEND = postgresql
     DB_PORT = {env:DB_PORT:5432}
 
-[testenv:py{36,37,38,39,310}-dj{22,31,32,40,main}-mysql]
+[testenv:py{36,37,38,39,310}-dj{22,32,40,main}-mysql]
 setenv =
     {[testenv]setenv}
     DB_BACKEND = mysql
     DB_PORT = {env:DB_PORT:3306}
 
-[testenv:py{36,37,38,39,310}-dj{22,31,32,40,main}-sqlite]
+[testenv:py{36,37,38,39,310}-dj{22,32,40,main}-sqlite]
 setenv =
     {[testenv]setenv}
     DB_BACKEND = sqlite3