It's just the same as with Java, GTK+, Qt or any other library/platform. If you're doing development with Java, you can use any version that you like (for example the version 1.5) - and your application probably won't be able to run by an older version of Java. The same is with, for example, GTK+ library - if you use the version 2.6, your application might not work on systems with older version of the GTK+ library.
And the same is with Python. When you develop application with Python, you must use some version of Python - and this is your dependency. You might say that your application needs a particular version of Python (and eventually particular versions of external modules). -- http://mail.python.org/mailman/listinfo/python-list
