Switch Black GitHub action to the official working version

[tburrows13]

For the last couple of days, our black GitHub action has been failing (/~https://github.com/Zulko/moviepy/runs/1222088083). This PR changes the action to one from the official black repo (/~https://github.com/psf/black/blob/master/Dockerfile). Note that it does not allow customisation of arguments, so without specifying Python 3.6 it defaults to "per-file auto-detection". I've tested this and no matter what version of Python 3 I specify, nothing changes in black's output. We'll continue to recommend that contributors use -t py36, so in the unlikely event that the GH action lets something incompatible with Python 3.6 through, someone else should catch it very quickly.

Issue reported to the original action: lgeiger/black-action#12
Feature request to official action: psf/black#1751

[coveralls]

Coverage remained the same at 67.861% when pulling 0fa188e on tburrows13:fix-black-action into 873530e on Zulko:master.