[Cython] By default, use same interpreter for mechanism conversions
[Cython] By default, use same interpreter for mechanism conversions

Fixes failures when only the Python 3 module is present.

git-svn-id: e76dbe14710aecee1ad27675521492cea2578c83@2668 02a645c2-efd0-11dd-984d-ab748d24aa7e

Cantera/cantera-svn,Cantera/cantera-svn,Cantera/cantera-svn,Cantera/cantera-svn,Cantera/cantera-svn,Cantera/cantera-svn