Add Django 3.0 testing to CI (#246)

* add Django 3.0 testing to CI

* remove importlib_metadata usage

* conditionally load choices module for tests

scripts/django_tests_settings.py

@@ -1,3 +1,5 @@
+import django
+
 SECRET_KEY = '1'
 SITE_ID = 1
 
@@ -41,7 +43,6 @@ test_modules = [
     'bulk_create',
     'cache',
     'check_framework',
-    'choices',
     'conditional_processing',
     'constraints',
     'contenttypes_tests',
@@ -219,6 +220,9 @@ test_modules = [
     'wsgi',
 ]
 
+if django.VERSION[0] == 2:
+    test_modules += ['choices']
+
 invalid_apps = {
     'import_error_package',
 }