Fix failing blacken-docs pre-commit hook

[hukkin]

No description provided.

[abravalheri]

Thank you very much @hukkin.

[abravalheri]

Is there any incompatibility with some versions, or you are just following the recommendations from the docs?

Now that black is becoming stable, with a clear update/deprecation policy, could we rely on the latest version?

[hukkin]

I'm just following the recommendation from the docs. Note that if we don't pin black version, then we should probably just delete this additional_dependencies line. black is required by blacken-docs anyways, just not version pinned.

Black's policy still allows formatting changes at the start of a new year, so one sensible alternative is pinning black==22.*.

[abravalheri]

Thank you very much for the clarification!

[abravalheri]

Thanks @hukkin, I merged your PR into master and I will adopt your suggestion about black==22.* in a separated commit.