Behind a proxy, `urllib` does not work with Python2 due to [Python issue
24599](https://bugs.python.org/issue24599).

Instead of using `urllib`, let's implement `urlretrieve` with `urllib2`, which
instead works and leave the Python3 implementation untouched (since that
one works too).

This fixes buildout issue #32.

There's no need to change the `try: except:IOError` block where `urlretrieve` is
used (line 195 in download.py) since the exceptions raised by `urllib2`
are subclasses of `IOError` and will then be caught.