Backport PR #2831: avoid string version comparisons in external.qt

We have learned from pyzmq that one should *always* parse version strings
before comparing them.
Backport PR #3013: py3 workaround for reload in cythonmagic

closes #3007
Backport PR #3008: fix cython module so extension for multiarched python

backported from a1daa0286a867cda86e728e177d72a645b72c634

see http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=697704