Add missing test for popitem() and fix bug in the C impl.

src/BTrees/BTreeTemplate.c

@@ -2091,7 +2091,7 @@ BTree_popitem(BTree* self, PyObject* args)
     }
 
     Py_DECREF(key);
-    return result_val;
+    return result;
 }
 
 

src/BTrees/BucketTemplate.c

@@ -1497,7 +1497,7 @@ bucket_popitem(Bucket* self, PyObject* args)
     }
 
     Py_DECREF(key);
-    return result_val;
+    return result;
 }
 
 

src/BTrees/tests/common.py

@@ -468,6 +468,23 @@ def _getCollectionsABC(self):
         from BTrees._compat import collections_abc
         return collections_abc.MutableMapping
 
+    def test_popitem(self):
+        t = self._makeOne()
+        # Empty
+        with self.assertRaises(KeyError):
+            t.popitem()
+
+        self._populate(t, 2000)
+        self.assertEqual(len(t), 2000)
+        for i in range(2000):
+            self.assertEqual(t.popitem(), (i, i))
+            self.assertEqual(len(t), 2000 - i - 1)
+
+        # Back to empty
+        self.assertEqual(len(t), 0)
+        with self.assertRaises(KeyError):
+            t.popitem()
+
     def testShortRepr(self):
         # test the repr because buckets have a complex repr implementation
         # internally the cutoff from a stack allocated buffer to a heap