Use channels 3 and django 3 if available

Author: fabienheureux

setup.cfg

@@ -55,8 +55,6 @@ test =
     mock; python_version<"3"
     django==1.11.*; python_version<"3"
     channels==1.*; python_version<"3"
-    django==2.*; python_version>="3"
-    channels==2.*; python_version>="3"
     django==3.*; python_version>="3"
     channels==3.*; python_version>="3"
     aiohttp; python_version>="3.5"