to listen on in network-oriented tests has been refined in an effort to
  facilitate running multiple instances of the entire regression test suite
  in parallel without issue.  test_support.bind_port() has been fixed such
  that it will always return a unique port -- which wasn't always the case
  with the previous implementation, especially if socket options had been
  set that affected address reuse (i.e. SO_REUSEADDR, SO_REUSEPORT).  The
  new implementation of bind_port() will actually raise an exception if it
  is passed an AF_INET/SOCK_STREAM socket with either the SO_REUSEADDR or
  SO_REUSEPORT socket option set.  Furthermore, if available, bind_port()
  will set the SO_EXCLUSIVEADDRUSE option on the socket it's been passed.
  This currently only applies to Windows.  This option prevents any other
  sockets from binding to the host/port we've bound to, thus removing the
  possibility of the 'non-deterministic' behaviour, as Microsoft puts it,
  that occurs when a second SOCK_STREAM socket binds and accepts to a
  host/port that's already been bound by another socket.  The optional
  preferred port parameter to bind_port() has been removed.  Under no
  circumstances should tests be hard coding ports!

  test_support.find_unused_port() has also been introduced, which will pass
  a temporary socket object to bind_port() in order to obtain an unused port.
  The temporary socket object is then closed and deleted, and the port is
  returned.  This method should only be used for obtaining an unused port
  in order to pass to an external program (i.e. the -accept [port] argument
  to openssl's s_server mode) or as a parameter to a server-oriented class
  that doesn't give you direct access to the underlying socket used.

  Finally, test_support.HOST has been introduced, which should be used for
  the host argument of any relevant socket calls (i.e. bind and connect).

  The following tests were updated to following the new conventions:
    test_socket, test_smtplib, test_asyncore, test_ssl, test_httplib,
    test_poplib, test_ftplib, test_telnetlib, test_socketserver,
    test_asynchat and test_socket_ssl.

  It is now possible for multiple instances of the regression test suite to
  run in parallel without issue.