[Ch 6] Add okpy.org to section on automated grading of assignments

[rwest]

As suggested on twitter by @natea, the section of chapter six on
Assessing student learning with Jupyter Notebooks
could mention https://okpy.org which says it "autogrades programming assignments, facilitates submission, composition feedback, and analytics for your class".
I've never used it so am not sure what to write, but the landing page does mention Jupyter Notebook Submission.

[lheagy]

@choldgraf also mentioned that data8 is considering using a somewhat new grading tool "Gopher Grader" https://github.com/data-8/Gofer-Grader

[choldgraf]

Yep - Gofer-Grader is basically a more tightly-scoped version of OKpy. It just does auto-grading on the same machine where the notebook exists (whereas OKpy is a service that does the grading for you). My intuition is that Gofer will be a better recommendation for people eventually, though not sure what its current state is.