Fix last commit changes in factparams.py for py2.7 and set the...

Fix last commit changes in factparams.py for py2.7 and set the pyfaust/matfaust.ConstraintGeneric class abstract.

For py2.7 the class is not abstract because it's complicated to do it compatibly in 2.7 and 3.6 so I privilegiate py3.6 code for the future.