[DO NOT MERGE] introduce isort and black as autoformatters

[pmeier]

Closes #4174. For now this is just a proof of concept PR. DO NOT MERGE this until we actually want to use black and isort.

Only pyproject.toml and setup.cfg need review. All the other changes were made by the autoformatters or to accommodate flake8.

Todo:

• Add black and isort to CI
• Add black and isort as pre-commit hooks

[pmeier]

Our current implementation relies on a specific import order in torchvision/ops/__init__.py since we have torchvision/ops/roi_align.py as well as a roi_align function. Thus, depending on the current stage of the imports from torchvision.ops import roi_align gives you either the module or the function.

In the future we should avoid name conflicts between modules and functions. For example, we could rename the module to _roi_align.py since all its functionality is available through torchvision.ops.

[NicolasHug]

Thanks @pmeier !

Considering the sequence of changes that will need to be done (in particular the doc updates, the pre-commit hook and the CI check), I would suggest to do all this in a separate branch first. We'll then be able to merge everything in master in one go, while keeping the development in separate PRs without worrying too much about the current state of master.

[pmeier]

Superseded by #4384.