Modify examples to be both Python 2 and 3 compatible

For removing dependency on 2to3

Examples/python/callback/runme.py

@@ -11,7 +11,7 @@ class PyCallback(example.Callback):
         example.Callback.__init__(self)
 
     def run(self):
-        print "PyCallback.run()"
+        print("PyCallback.run()")
 
 # Create an Caller instance
 
@@ -20,8 +20,8 @@ caller = example.Caller()
 # Add a simple C++ callback (caller owns the callback, so
 # we disown it first by clearing the .thisown flag).
 
-print "Adding and calling a normal C++ callback"
-print "----------------------------------------"
+print("Adding and calling a normal C++ callback")
+print("----------------------------------------")
 
 callback = example.Callback()
 callback.thisown = 0
@@ -29,9 +29,9 @@ caller.setCallback(callback)
 caller.call()
 caller.delCallback()
 
-print
-print "Adding and calling a Python callback"
-print "------------------------------------"
+print("")
+print("Adding and calling a Python callback")
+print("------------------------------------")
 
 # Add a Python callback (caller owns the callback, so we
 # disown it first by calling __disown__).
@@ -40,9 +40,9 @@ caller.setCallback(PyCallback().__disown__())
 caller.call()
 caller.delCallback()
 
-print
-print "Adding and calling another Python callback"
-print "------------------------------------------"
+print("")
+print("Adding and calling another Python callback")
+print("------------------------------------------")
 
 # Let's do the same but use the weak reference this time.
 
@@ -53,5 +53,5 @@ caller.delCallback()
 
 # All done.
 
-print
-print "python exit"
+print("")
+print("python exit")