Python Client [v2.X] Added inheritance/polymorphism (allOf) to python client

[mjschuetze102]

PR checklist

• [x] Read the contribution guidelines.
• [x] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• [x] Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• [x] Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

This takes some of the changes made to OpenAPI v3.0 and expands on them to add inheritance/ polymorphism to Swagger 2.0

Fixes: #3904, #10912, #11225 Similar fixes could be applied to solve several issues for v3.X

[jcgentr]

Can this be merged? This would be extremely helpful for Python devs using this tool.