chore(deps): update testing pip deps (minor)

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
flake8 (changelog)	==6.0.0 -> ==6.1.0
pytest-operator	==0.28.0 -> ==0.29.0

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Release Notes

pycqa/flake8 (flake8)

### [`v6.1.0`](https://togithub.com/pycqa/flake8/compare/6.0.0...6.1.0) [Compare Source](https://togithub.com/pycqa/flake8/compare/6.0.0...6.1.0)

charmed-kubernetes/pytest-operator (pytest-operator)

### [`v0.29.0`](https://togithub.com/charmed-kubernetes/pytest-operator/compare/0.28.0...0.29.0) [Compare Source](https://togithub.com/charmed-kubernetes/pytest-operator/compare/0.28.0...0.29.0)

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