[python-flask] adopt python3.5+ syntax

[anis-campos]

removing some residual python2 code, since it is not supported anymore, like:

• no need for six anymore
• no need for encoding utf8 in top file
• remove object inheritance in base model
• remove absolute import __future__

resolve #16377

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[wing328]

FYI. I've filed #16321 to add better pydantic support in the python fastapi generator. Do you think python-flask generator also needs similar enhancement (python flask generator does not use pydantic at the moment if I remember correctly) ?

[anis-campos]

FYI. I've filed #16321 to add better pydantic support in the python fastapi generator. Do you think python-flask generator also needs similar enhancement (python flask generator does not use pydantic at the moment if I remember correctly) ?

I'm not a user of pydantic, we generally use marshmallow and dataclasses in our projects, so I can't really tell the pros and cons, but I will look up to it, it may do some good for us.

However, I'm not sure that adding support for pydantic in the flask generator at this point is really wise. IMO, I would rather have it in another generator, or alternatively, it should be a choice at generation time, not sure how it would pan out, but it would certainly be a breaking change for those already using it.

[wing328]

thanks for the feedback. we won't add it to the python flask generator for the time being.

[wing328]

LGTM. Thanks for the PR.

FYI @taxpon (2017/07) @frol (2017/07) @mbohlool (2017/07) @cbornet (2017/09) @kenjones-cisco (2017/11) @tomplus (2018/10) @arun-nalla (2019/11) @krjakbrjak (2023/02)