A py.test plugin that stores test expectations by saving the set of
failing tests, allowing them to be marked as xfail when running them
in future. The tests expectations are stored such that they can be
distributed alongside the tests. However, note that test expectations
can only be reliably shared between Python 2 and Python 3 if they only
use ASCII characters in their node ids: this likely isn’t a limitation
if tests are using the normal Python format, as Python 2 only allows
ASCII characters in identifiers.