Add ipdb to test deps

This is not strictly necessary but it helps tremendously to have a REPL
around to break into when a test fails.

tox.ini

@@ -12,6 +12,7 @@ setenv =
 	postgres: DEFAULT_DATABASE = postgres:///default
 	postgres: SECONDARY_DATABASE = postgres:///secondary
 deps =
+	ipdb
 	coverage
 	dj-database-url
 	django21: Django ~= 2.1