Replace backticks with repr() or "%r"

From SF patch #852334.
diff --git a/Demo/sockets/unixclient.py b/Demo/sockets/unixclient.py
index a0d80f6..cccd617 100644
--- a/Demo/sockets/unixclient.py
+++ b/Demo/sockets/unixclient.py
@@ -7,4 +7,4 @@
 s.send('Hello, world')
 data = s.recv(1024)
 s.close()
-print 'Received', `data`
+print 'Received', repr(data)