Explore the possibility of type hinting

Python 3 has a type hinting feature very useful for auto complete. This issue is for tracking efforts on researching its backward compatibility with python 2 (`from __future__ import import annotations`?) and if possible, change code generator to implement it. Best if we contribute it back to swagger-codegen.

Start by reviewing the existing code generator and the Python 2 compatibility constraints described in the issue, then compare the proposed annotation approach with swagger-codegen. Done means the compatibility question and implementation scope are resolved, with any feasible change prepared for contribution upstream.

Written by the indexing model from the issue text.