Merge: #10883: Fix socket leaks in urllib.request.

* ftpwrapper now uses reference counting to ensure that the underlying socket
  is closed when the ftpwrapper object is no longer in use
* ftplib.FTP.ntransfercmd() now closes the socket if an error occurs

Initial patch by Victor Stinner.

Lib/test/test_urllib2.py

@@ -623,6 +623,7 @@ class HandlerTests(unittest.TestCase):
             def retrfile(self, filename, filetype):
                 self.filename, self.filetype = filename, filetype
                 return io.StringIO(self.data), len(self.data)
+            def close(self): pass
 
         class NullFTPHandler(urllib.request.FTPHandler):
             def __init__(self, data): self.data = data