Remove assert_called_once in tests

[mzjp2]

cf PyCQA/pycodestyle#624 👨‍💻

Don't think this is worth adding myself to RELEASE.md, feel free to merge/close. 🎸

Notice

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Motivation and Context

Using assert_called_once isn't recommended, especially when supporting Python 3.5, since it doesn't exist, but because the object is mocked, no error is raised.

How has this been tested?

🤖

Checklist

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change and added my name to the list of supporting contributions in the RELEASE.md file
• Added tests to cover my changes
• Assigned myself to the PR

[921kiyo]

Thanks 👍

[lorenabalan]

Nice one!