In addition to DM-2492, we've found another unguarded import of matplotlib.pyplot, which can apparently cause problems on some (but not all) display-less systems.
prepare v10_1_rc3 release candidate
shapelet unit tests attempts to access display on failure
Fix confirmed by wmwood-vasey; since this is a trivial removal of an unnecessary line, I'll count that as the review.
The next release candidate should include this patch, which is trivial and has zero chance of causing any new problems.