Clean-up - Unnecessary trailing underscores in Python Flask, AIOHttp, BluePlanet

[yilmi]

Description

Some of the files generated for the python-flask language contain unnecessary trailing underscores at the end of their name.

I looked for a good reason to do so, but couldn't find one, these happened in:

• Python AIOHTTP server generator #1470
• Some of the swagger code

I have renamed the files to have them named base_model.py (instead of base_model_.py) and security_controller.py (instead of security_controller_.py). While doing so, I had to touch the AbstractPythonConnexionServerCodegen which ended up touching the AIOHttp and BluePlanet Python generators, which I also fixed.

Adding @krjakbrjak and @spacether as Python committee member for review and feedback

Merging into master as it is a minor change.

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[wing328]

I didn't know the reason either. Let's go with your change and see how that goes.