PEP 479: Change StopIteration handling inside generators.

Closes issue #22906.
diff --git a/Python/future.c b/Python/future.c
index 81eab54..163f87f 100644
--- a/Python/future.c
+++ b/Python/future.c
@@ -40,6 +40,8 @@
             continue;
         } else if (strcmp(feature, FUTURE_BARRY_AS_BDFL) == 0) {
             ff->ff_features |= CO_FUTURE_BARRY_AS_BDFL;
+        } else if (strcmp(feature, FUTURE_GENERATOR_STOP) == 0) {
+            ff->ff_features |= CO_FUTURE_GENERATOR_STOP;
         } else if (strcmp(feature, "braces") == 0) {
             PyErr_SetString(PyExc_SyntaxError,
                             "not a chance");