mpi4py: use old code with Python 2.6 as well

Seems to have issues on OS X despite it working with 2.7 and the symbol
being in both.

Change-Id: If0d8e8f98491e6b024c9794b260a8545b8a8d554
Merge topic 'mpi4py-osx' into master

0fe880d6 mpi4py: use old code with Python 2.6 as well