The suggested sensor add-on board for this course actually uses a different command set to the SHT21 and SHT11 (my bad, I shouldn't have assumed).

So, there is a new branch on the Git repo called "sht31" that contains example code for use with the SHT31. I don't have an SHT31 handy to test with, so if you find any bugs, feel free to submit pull requests, or just file it as an issue and I will fix it!

Apologies to those who were wondering why the code wasn't working - this is all on me.