Add support for @flaky.

@genty needs to be aware of @flaky (see github.com/box/flaky). If
a test method is decorated by @flaky, the hidden 'flaky' data needs
to be deepcopied into each of the genty generated test methods.

This commit doesn't include test coverage because @flaky isn't
public yet. Once flaky is public and on pypi the test associated
with this change will be pushed.