Add "generator_stop" to default future imports for language_level=3.

See #2580.

Cython/Compiler/Main.py

@@ -98,16 +98,16 @@ class Context(object):
         self.gdb_debug_outputwriter = None
 
     def set_language_level(self, level):
-        from .Future import print_function, unicode_literals, absolute_import, division
+        from .Future import print_function, unicode_literals, absolute_import, division, generator_stop
         future_directives = []
         if level == '3str':
-            future_directives = [print_function, absolute_import, division]
+            future_directives = [print_function, absolute_import, division, generator_stop]
             self.future_directives.discard(unicode_literals)
             level = 3
         else:
             level = int(level)
             if level >= 3:
-                future_directives = [print_function, unicode_literals, absolute_import, division]
+                future_directives = [print_function, unicode_literals, absolute_import, division, generator_stop]
         self.language_level = level
         if future_directives:
             self.future_directives.update(future_directives)