The scope system changed between Python 2 and 3 on some aspects and we should let the user configure the version to have correct results.

For example, in Python 2 a list comprehension did not had a scope level, thus the variable declared in the comprehension leaked after its expression. In Python 3, the comprehension has its own scope preventing a leakage.

For now, I'll manage this as if we knew the model is in Python 3. But we should probably add support for Python 3 later.