[Python] Avoid DeprecationWarning in inspect.getargspec on Python 3

[johnthagen]

Closes #9270

The change is in modules/openapi-generator/src/main/resources/python-legacy/model.mustache.

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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[johnthagen]

Mentioning Python technical committee as recommended: @spacether @arun-nalla @Jyhess @tomplus @kenjones-cisco @cbornet @mbohlool @frol @taxpon

[spacether]

This looks good; thank you for your PR!