free cart pendulum animation example

[zjor]

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[ImportanceOfBeingErnest]

Thanks for your pull request. Unfortunately you left the description empty. Can you motivate what you want to add here and why?
There is already an example in the gallery showing a pendulum animation. In how far is this different? Why would we add this example? Does it show anything that is missing in the other example?

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On a more formal level, note that `scipy` is not a dependency of matplotlib, so we cannot simply use it as such in examples.

[jklymak]

@zjor thanks for the PR, but I'm inclined to agree w/ @ImportanceOfBeingErnest . Closing for now, but if there is more to this than there appears, then let us know and we can re-open!