Formatting doctests in doc strings

[ChrisWellsWood]

I often find myself writing a doctest, pasting it into a separate file, formatting it with Black then pasting it back into the doc string. It'd be very convenient if Black formatted the code in doctests, maybe it could be behind a flag.

I saw @ambv saying that doc strings are out of scope in another issue (#144 (comment)) but I see less controversy about formatting the doctests as they're code.

[mattharrison]

I wanted to see how applying black to the code (written as doctests) would look in my upcoming book. So I wrote a script to do it here.

https://gist.github.com/mattharrison/2a1a263597d80e99cf85e898b800ec32

[ambv]

We'll decide whether to do this as part of #144.