[Python] Use Python 2.6 compatible dict definition

[richardwhiuk]

This changes the generated client to use a Python 2.6 compatible dictionary definition, which allows the client (on at least some APIs) to work on Python 2.6

This work was contributed by @nick-slack at @Metaswitch.

Python Technical Committee

@taxpon, @frol, @mbohlool, @cbornet, @kenjones-cisco, @tomplus, @Jyhess, @slash-arun

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

[spacether]

Python 2.7 end of life was Jan 1, 2020 per python/devguide#344
For now we will limit our syntax to 2.7 and higher syntax
If users want to use older versions of python with the generator, then they can customize the templates per @jimschubert's suggestion

In this issue: #4933
A plan has been laid out to remove python 2.X support and switch to Python3 only ~July 2020