CP-49148: Convert rrdd-example.py to python3

[stephenchengCloud]

Since this is only an example file, no xenrt test.
This file is not installed on dom0, should do no harm.

• pylint and pytype check passed
• Manually tested the main logic works in python3:

Python 3.11.0 (main, Jun 21 2024, 03:47:37) [GCC 13.3.1 20240522 (Red Hat 13.3.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> cmd = "free -k | grep Mem | awk '{print $2, $3, $4}'"
>>> import os
>>> vs = os.popen(cmd).read().strip().split()
>>> vs
['16336416', '4351784', '1958616']