Add codecov dependency

Author: davesque

tox.ini

@@ -35,6 +35,7 @@ deps=
     drf38: djangorestframework>=3.8,<3.9
     drf39: djangorestframework>=3.9,<3.10
     drf310: djangorestframework>=3.10,<3.11
+    py37-django22-drf310: codecov
     djangomaster: https://github.com/django/django/archive/master.tar.gz
 whitelist_externals=make