patch 8.2.0557: no IPv6 support for channels

Problem:    No IPv6 support for channels.
Solution:   Add IPv6 support. (Ozaki Kiichi, closes #5893)

src/testdir/test_channel_6.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python
+#
+# Server that will accept connections from a Vim channel.
+# Used by test_channel.vim.
+#
+# This requires Python 2.6 or later.
+
+from test_channel import main, ThreadedTCPServer
+import socket
+
+class ThreadedTCP6Server(ThreadedTCPServer):
+    address_family = socket.AF_INET6
+
+if __name__ == "__main__":
+    main("::", 0, ThreadedTCP6Server)