MAINT: pin matplotlib version

The plotting in the CI suddenly stopped after the image updated the matplotlib version. We need to get this in to fix the CI to make the next release. I'm not able to reproduce the error on my mac, but we can revisit why this is failing after getting the release out.

changed milestone to %2.2.0

@simon-stevenson please can you give an approval for this as it will make it into the next planned release?

added <10 lines Infrastructure labels

There has been a related discussion at https://github.com/dfm/corner.py/issues/236.

requested review from @simon-stevenson

assigned to @simon-stevenson

approved this merge request

approved this merge request

merged

mentioned in merge request pesummary!738 (merged)

mentioned in merge request !1338 (merged)