Bug#936999: pymvpa2: Python2 removal in sid/bullseye

[Matthias Klose]

Control: severity -1 serious

the mdp and pymvpa2 packages are not in testing (both for more than a year), and 
now blocking some upgrade issues for Python3.8.  We will ignore these packages, 
and start removing their python2 dependencies, starting with scikit-learn.

I'm preparing a NMU for scikit-learn, based on the new subminor upstream release 
0.20.3, not looking at 0.21.

Matthias