Upgrade pre-commit flake8 to 6.0.0

[mzweilin]

Upgrade pre-commit flake8 to 6.0.0 to work with newer importlib_metadata.

Otherwise, the code quality PR check fails.

I picked flake8 v6.0.0 following the choice of the template:

https://github.com/ashleve/lightning-hydra-template/blob/bddbc24b82ab6ccfa6243e815a49dc5bfe8d4144/.pre-commit-config.yaml#L73C10-L73C15

What does this PR do?

Fixes #\

Type of change

Please check all relevant options.

• [ ] Improvement (non-breaking)
• [x] Bug fix (non-breaking)
• [ ] New feature (non-breaking)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Testing

Please describe the tests that you ran to verify your changes. Consider listing any relevant details of your test configuration.

• [ ] pytest
• [ ] CUDA_VISIBLE_DEVICES=0 python -m mart experiment=CIFAR10_CNN_Adv trainer=gpu trainer.precision=16 reports 70% (21 sec/epoch).
• [ ] CUDA_VISIBLE_DEVICES=0,1 python -m mart experiment=CIFAR10_CNN_Adv trainer=ddp trainer.precision=16 trainer.devices=2 model.optimizer.lr=0.2 trainer.max_steps=2925 datamodule.ims_per_batch=256 datamodule.world_size=2 reports 70% (14 sec/epoch).

Before submitting

• [x] The title is self-explanatory and the description concisely explains the PR
• [x] My PR does only one thing, instead of bundling different changes together
• [ ] I list all the breaking changes introduced by this pull request
• [ ] I have commented my code
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] I have run pre-commit hooks with pre-commit run -a command without errors

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