Fix tests for python >= 3.10

[thebaptiste]

nose is not python 3.10 compliant
We should use nose2 or pytest

[amotl]

Dear Jean-Baptiste,

I would like to recommend switching over to pytest, I also did it recently at ip-tools/patzilla@8ad073781. However, when just swapping nose to nose2 is more effortless, I can understand that this might be more straight-forward.

With kind regards,
Andreas.

[thefab]

+1 for pytest :)