py: Fix with+for+return bug by popping for-iter when unwinding exc stack.

Addresses issue #1182.

tests/basics/with_return.py

@@ -1,14 +1,53 @@
 class CtxMgr:
+    def __init__(self, id):
+        self.id = id
 
     def __enter__(self):
-        print("__enter__")
+        print("__enter__", self.id)
         return self
 
     def __exit__(self, a, b, c):
-        print("__exit__", repr(a), repr(b))
+        print("__exit__", self.id, repr(a), repr(b))
 
+# simple case
 def foo():
-    with CtxMgr():
+    with CtxMgr(1):
         return 4
-
 print(foo())
+
+# for loop within with (iterator needs removing upon return)
+def f():
+    with CtxMgr(1):
+        for i in [1, 2]:
+            return i
+print(f())
+
+# multiple for loops within with
+def f():
+    with CtxMgr(1):
+        for i in [1, 2]:
+            for j in [3, 4]:
+                return (i, j)
+print(f())
+
+# multiple for loops within nested withs
+def f():
+    with CtxMgr(1):
+        for i in [1, 2]:
+            for j in [3, 4]:
+                with CtxMgr(2):
+                    for k in [5, 6]:
+                        for l in [7, 8]:
+                            return (i, j, k, l)
+print(f())
+
+# multiple for loops that are optimised, and nested withs
+def f():
+    with CtxMgr(1):
+        for i in range(1, 3):
+            for j in range(3, 5):
+                with CtxMgr(2):
+                    for k in range(5, 7):
+                        for l in range(7, 9):
+                            return (i, j, k, l)
+print(f())