PEP 479: Change StopIteration handling inside generators.

Closes issue #22906.
diff --git a/Include/pythonrun.h b/Include/pythonrun.h
index 1d9b71b..f92148d 100644
--- a/Include/pythonrun.h
+++ b/Include/pythonrun.h
@@ -9,7 +9,8 @@
 
 #define PyCF_MASK (CO_FUTURE_DIVISION | CO_FUTURE_ABSOLUTE_IMPORT | \
                    CO_FUTURE_WITH_STATEMENT | CO_FUTURE_PRINT_FUNCTION | \
-                   CO_FUTURE_UNICODE_LITERALS | CO_FUTURE_BARRY_AS_BDFL)
+                   CO_FUTURE_UNICODE_LITERALS | CO_FUTURE_BARRY_AS_BDFL | \
+                   CO_FUTURE_GENERATOR_STOP)
 #define PyCF_MASK_OBSOLETE (CO_NESTED)
 #define PyCF_SOURCE_IS_UTF8  0x0100
 #define PyCF_DONT_IMPLY_DEDENT 0x0200