Don't include read-only properties in Python examples.

[therve]

When using a schema with read-only fields as API inputs, Python
generates examples for those: this excludes them.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
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[therve]

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

[spacether]

@therve these examples should only be omitted when sending data to the server, right?
One could get back a readonly property or instance, right?
Our tooling does not currently autogen mock responses but if we did in the future one would need to be aware of this.

[therve]

@therve these examples should only be omitted when sending data to the server, right?
One could get back a readonly property or instance, right?
Our tooling does not currently autogen mock responses but if we did in the future one would need to be aware of this.

Right if we ever generate response code this would need to be changed.

[spacether]

Looks good; thank you for your PR!