Turn off pre-commit's automatic parallelization for black

[asottile]

black internally uses multiprocessing for speed. In pre-commit 1.13.0 this is automated by the framework itself however if both pre-commit and black are forking processes this is slower and hits race-conditions in black.

[coveralls]

Pull Request Test Coverage Report for Build 868

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.65%

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Totals
Change from base Build 865:	0.0%
Covered Lines:	2788
Relevant Lines:	3042

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💛 - Coveralls

[zsol]

Thanks!

[asottile]

@zsol mind taking a look at #674 (oops had the wrong number here) as well? it's related :D

[zsol]

I did actually, just didn't have a chance to play around with it. Anyway I saw Jelle already merged it. Thanks for that, too :)