Add CI to check that code has been formatted with pyink formatter.

[alxmrs]

          One other thing I'll note is that I've been formatting code in Xarray-Beam with the (Google internal) version of the "pyink" formatter, using 2 space indentation: https://github.com/google/pyink

It would be nice if you could apply that manually to this change -- ideally we would add some sort of automatic formatter checker.

Originally posted by @shoyer in #69 (comment)