Fix test_ftplib warning if IPv6 is not available (#1457)

Fix test_ftplib warning if IPv6 is not available (#1457)

DummyFTPServer now calls del_channel() on bind() error to prevent the
following warning in TestIPv6Environment.setUpClass():

Warning -- asyncore.socket_map was modified by test_ftplib
  Before: {}
  After:  {3: <test.test_ftplib.DummyFTPServer 127.0.0.1:0 at ...>}