@platteau: External methods still work, but aren't well supported by documentation. BrowserViews are unrestricted (as is all filesystem) code and often preferred by experienced developers as its easier to deploy in a package. But there is more to learn, for example python packaging.
http://docs.plone.org/develop/plone/views/browserviews.html#creating-and-registering-a-view
But there are tutorials to help you.
BTW, there is one other possibility. If the library you want to access from the ZMI is common, I've been working on a package you can install to allow many commonly request imports in python scripts. https://github.com/collective/collective.trustedimports. Still new but please raise a ticket if the code you want to access should be included.