Generally my shell is in the tests directory, and there was no sign that fix
wasn't doing anything, even with --debug and without --quiet on the grey
invocation. (Maybe it would be useful to state when no files were processed?)
I only tested the black config, but the others are similar enough.
The need for a different command based on the platform is a bit unfortunate. I
wonder if it's worth a {python} keyword for portability, since the python
binary isn't in PATH by default on Windows (IIRC), and doesn't have the version
number in any case.