the change that caused it:
https://github.com/dropbox/mypy-protobuf/issues/118
This is more awkward to handle now, as the types are only available
at type-checking time. Python's static typing is such a mess :-(
This allows us to add a docstring to .append() so users can see
the names of the arguments that are being passed, and means we
don't have to remember to prepend run_ when calling a hook.
pytest will show what differs in simple assert statements
concurrent mode is supported with a plugin, but like nose2, concurrent
mode hides the cause of import errors, so I've left it off for now.
