Add Python 3 required statement

[elliotwutingfeng]

From #740, Python 2 is no longer supported. This line python update_plugins.py # use python3 if python is unavailable makes it sound like this script still supports Python 2. So a Python 3 is required. statement clears up that ambiguity.

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The current update_plugins.py script will not run on Python 2 because of the use of urllib.request.urlopen (Python 2 uses urllib2 module for the urlopen method), and in Python 2, urllib2.urlopen needs contextlib.closing to be used in a with clause.