Add Python 3.9 support to test suite

https://docs.python.org/3.9/whatsnew/3.9.html

Author: Rust Saiargaliev

.github/workflows/ci.yml

@@ -36,6 +36,7 @@ jobs:
           - 3.6
           - 3.7
           - 3.8
+          - 3.9
         django-version:
           - 2.2.*
           - 3.1.*

setup.cfg

@@ -23,6 +23,7 @@ classifier =
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
     Framework :: Django
     Framework :: Django :: 2.2
     Framework :: Django :: 3.1