BF(PY3): use next() instead of .next() (PY3 only)

Author: yarikoptic

chain.py

@@ -57,7 +57,7 @@ def delegate(self, requests):
 def coroutine(func):
     def start(*args, **kwargs):
         cr = func(*args, **kwargs)
-        cr.next()
+        next(cr)
         return cr
     return start