Use of variable type if discrimator is not Python 3.5 valid

[lmazuel]

Example:

autorest.python/test/vanilla/Expected/AcceptanceTests/BodyComplex/bodycomplex/models/_models_py3.py

Line 221 in b31da9a

self.fishtype: Optional[str] = None

This likely mean that on Python 3.5, we fallback on the 2.7 models right now. Use #type instead

We should have an assert in each test to check that if Python 3.x, we loaded the Py3 files (that would have failed the CI)