Add an example of a generator->freevar->cell->generator reference-cycle that

doesn't get cleaned up and thus leaks.

Lib/test/leakers/test_generator_cycle.py

@@ -0,0 +1,11 @@
+
+# This leaks since the introduction of yield-expr and the use of generators
+# as coroutines, trunk revision 39239. The cycle-GC doesn't seem to pick up
+# the cycle, or decides it can't clean it up.
+
+def leak():
+    def gen():
+        while True:
+            yield g
+    g = gen()
+