Issue with one test on python 2.7 and 3.5

[smarie]

https://github.com/python-qds/qdscreen/actions/runs/4441558867/jobs/7796796417?pr=41#step:6:101

We should investigate this later, maybe it is worth understanding this as it seems related to our "nan policy", see #34

For now I'll mark the test as xfail