[Cython] Fix syntax errors in MixMaster for Python 3
[Cython] Fix syntax errors in MixMaster for Python 3

Backticks are not allowed in Python 3 (equivalent to repr)


imitrichev/cantera,Heathckliff/cantera,imitrichev/cantera,imitrichev/cantera,Heathckliff/cantera,Heathckliff/cantera,imitrichev/cantera,imitrichev/cantera,imitrichev/cantera,Heathckliff/cantera,Heathckliff/cantera,Heathckliff/cantera