BUG: incompatibility with corner 2.2.2

[neutrinoceros]

amical/tests/test_analysis.py::test_pymask_mcmc is currently failing (see https://github.com/SAIL-Labs/AMICAL/actions/runs/4643055517/jobs/8218550363) with the following error

ValueError: 'title_quantiles' must contain exactly three values; pass a length-3 list or array using the 'title_quantiles' argument

This is caused by an intentional change between corner==2.2.1 and corner==2.2.2 (just released). See dfm/corner.py#193

It's a little unsettling that a bugfix release breaks us (I would have expected a warning), but it also seems pretty easy to fix on our side so let's try that first.