Align black config with Haystack repo

[anakin87]

In #4, I said:

If I try to run pre-commit run --all-files, it seems that black-jupyter (22.6.0) wants to reformat all the notebooks.

@masci:

we never run a formatter on the notebooks, I think this is a good opportunity to do it. The only problem I see, it's already hard to port changes from the haystack repo now, if we re-format the files it'll be impossible. So I would re-format all the things only if we're close enough to remove the files from the haystack repo. WDYT?

@masci @TuanaCelik I found out what the problem is:
in Haystack repo we are running black with the configurations specified in pyproject.toml:

[tool.black]
line-length = 120
skip_magic_trailing_comma = true  # For compatibility with pydoc>=4.6, check if still needed.

Proposed changes

In this PR, I fix this misalignment, by adding these parameters to the black entry in .pre-commit-config.yaml
Another option could be to add pyproject.toml to this repo: maybe it is premature or not wanted/needed.

[masci]

I'd add a pyproject file to configure black, so we can reuse it in the CI

[anakin87]

@masci I added a minimal pyproject file, based on that of the main repo.
Feel free to review/edit it.

[masci]

Thanks!