buildout for running tests in Plone 3.3.x fails

when running the buildout for tests under Plone 3.3.x, it ends with the following error:

```
SyntaxError: invalid syntax
An error occured when trying to install zope.interface 4.0.1. Look above this message for any errors that were output by easy_install.
While:
Installing.
Getting section test.
Initializing section test.
Installing recipe zc.recipe.testrunner.
Getting distribution for 'zope.interface'.
Error: Couldn't install: zope.interface 4.0.1
```

this could be some issue on my machine, but I'm not sure.