[python] Add enum values as class attributes to enum models

[phillipuniverse]

Fixes #10752

This adds all of the enum values as class attributes onto enum models so that they can be accessed in a typesafe way.

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@taxpon @frol @mbohlool @cbornet @kenjones-cisco @tomplus @Jyhess @arun-nalla @spacethe

[phillipuniverse]

Well this won't work, maybe we'll need a different solution for special characters.

[MalteEbner]

I have the same problem, do you have an estimate when this is going to be merged?

Please also note that the current PR adds newlines to many files. Would it be possible to only add the newline if there is a enum and otherwise not?

[phillipuniverse]

Seems like there is not much appetite to do much else for this PR in this Python client. Seems like newer functionality is going into the python experimental branch.

In the meantime, the workaround if you need this functionality is to:

1. Create a folder to hold custom templates and overrides (e.g. python-codegen-templates)
2. Create a folder called model_templates
3. Copy in the modified classvars.mustache in this PR into model_templates, so the path would be python-codegen-templates/model_templates/classvars.mustache
4. Generate with the --templates-dir=python-codegen-templates/ argument. Here's an example using the OpenAPI generator Docker image:

docker run --rm --user "$(id -u)":"$(id -g)" \
    -v "$(pwd)":/local \
    openapitools/openapi-generator-cli:$OPENAPI_GENERATOR_VERSION generate \
    -g python \
    -i /local/"$OPENAPI_FILE" \
    -o /local/"$OUTPUT_DIR" \
    --template-dir=/local/python-codegen-templates \
    --additional-properties=packageName="$PACKAGE_NAME",packageVersion="$SCHEMA_VERSION",projectName="$PROJECT_NAME",pythonAttrNoneIfUnset=true