Add Python3.9 support.

Author: hramezani

.github/workflows/test.yml

@@ -146,7 +146,7 @@ jobs:
       fail-fast: false
       max-parallel: 5
       matrix:
-        python-version: ['3.6', '3.7', '3.8']
+        python-version: ['3.6', '3.7', '3.8', '3.9']
 
     steps:
     - uses: actions/checkout@v2

tox.ini

@@ -3,10 +3,7 @@ envlist =
     docs
     style
     readme
-    py{36,37,38}-dj22-sqlite
-    py{36,37,38}-dj{30,31}-sqlite
-    py{36,37,38}-djmaster-sqlite
-    py{37,38}-dj{22,30,31}-{postgresql,mysql}
+    py{36,37,38,39}-dj{22,30,31,master}-{sqlite,postgresql,mysql}
 
 [testenv]
 deps =
@@ -63,6 +60,7 @@ python =
     3.6: py36
     3.7: py37
     3.8: py38
+    3.9: py39
 
 [gh-actions:env]
 DB_BACKEND =