Avoid leaving cyclic trash around.

src/ZODB/tests/testmvcc.py

@@ -224,6 +224,10 @@ cache, so our first reference to it doesn't return a ghost.
 >>> r1["b"]._p_state
 -1
 
+>>> cn1._transaction = None
+
+(See the Cleanup section below.)
+
 Late invalidation
 -----------------
 
@@ -301,7 +305,6 @@ Connection and Transaction.  The Transaction has an __del__ method,
 which prevents the cycle from being collected.  There's no API for
 clearing the Connection's local transaction.
 
->>> cn._transaction = None
 >>> cn1._transaction = None
 >>> cn2._transaction = None