Fix bug in urllib3 version comparison

If urllib_ver is 'dev', `StrictVersion()` will throw an exception. The check should ensure that it is not dev.
what does the ``if urllib3`` accomplish? It will always be there (and it was already used when extracting ``urllib_ver``. If it were not available at all, there would be an ``ImportError``

Maybe it should be ``if urllib_ver`` for the cases where that ends up being an empty string?