BF(TST): compatibility with numpy <= 1.4.1 -- use straight sorted on heterogeneous lists

Detected on nibotmi buildbot http://nipy.bic.berkeley.edu/builders/pymvpa-py2.6-osx-10.5-ppc/builds/153/steps/shell_6/logs/stdio
I left python3 logic previous (np.sort) since it is unlikely there would be elderly numpy
tested with python3 anyways, so minimized the change