Log message:
py-backcall: pytest from versioned depends

Log message:
py-backcall: added version 0.1.0

Specifications for callback functions passed in to an API

If your code lets other people supply callback functions, it's important to
specify the function signature you expect, and check that functions support
that. Adding extra parameters later would break other peoples code unless
you're careful.

backcall provides a way of specifying the callback signature using a prototype
function.