Move development dependencies to pyproject.toml

[sbrugman]

Minor PR to kick-off contributing to torchfix. I'm developing new functionality, which I plan to contribute in follow-up PRs.

Changes:

• Move requirements-dev.txt to optional-dependencies
• Explicitly set the setuptools build system
• Omit pip install pre-commit in CONTRIBUTING.md, already included in dev dependencies

[kit1980]

Thanks!
Something is not right, as pytest is now not found.

[sbrugman]

Thanks for the quick reply! Was able to resolve the issues with the CI/CD feedback:

• Ensured the build dependencies are up-to-date
• Added the __init__.py file to torch.visitors to make it a proper namespace package

[sbrugman]

Thanks!