Run CI tests with SPAMS 2.6 on Python 2 and 3
Run CI tests with SPAMS 2.6 on Python 2 and 3

As SPAMS 2.6 supports Python 3, add it into the matrix for Python 3 as
well. This should also touch some code paths that aren't really tested
on Python 3 currently. So should improve code quality as well.

nanshe-org/nanshe,DudLab/nanshe,nanshe-org/nanshe,jakirkham/nanshe,DudLab/nanshe,jakirkham/nanshe