Update black instructions

[max-sixty]

This needed updating with the relevant commit post-black change

I also added a line to apply the patch of manual changes we made on top of the black changes. It makes the list of steps a bit burdensome. But I've tested them a couple of times and, where people have lots of code changes, it's still going to be much easier than resolving manually.

I generally wouldn't want to suggest people curl data from the internet (even if we trust the individual). I think it's probably OK in this instance: the address contains a hash of the contents, and it's only being fed into git apply -, not executed. But lmk if that's still a security concern.

I'll update the issue I opened on black with the above issue; we should have done one commit with only the black changes, and then another with any manual changes.

[dcherian]

These instructions worked quite well on #3175

[max-sixty]

Great - thanks for the update @dcherian !