remove specification of language_version on black pre commit hook

[RonnyPfannschmidt]

followup to #3839

once black does a release which allows to use any python3 in turn enabling python3.7 and newer we should drop our mirror of their setup

[asottile]

Note that blacken-docs v0.2.0-1 allows this already 🎉

[RonnyPfannschmidt]

@asottile great job 👍

[wimglenn]

pytest/tox.ini

Line 41 in 43657f2

basepython = python3.6

[RonnyPfannschmidt]

@wimglenn thanks for that note - for that one i actually want to open a tox issue