Bug#931540: transition: statsmodels 0.8 -> 0.9 or 0.10

[Rebecca N. Palmer]

0.9 in salsa is now buildable (but _not_ ready for upload).  With this 
version installed:
seaborn, tnseq-transit: build successful
pymvpa2: the same FTBFS as with 0.8 (#931888)

pandas only build-depends on statsmodels for documentation examples (run 
during build), and the others import (somewhere in their source, which 
might be in an example) but don't (build-/autopkgtest-)depend.

Python 2 can't be dropped at this point, as tnseq-transit and pymvpa2 
are Python 2 only hard (Build-)Depends.  (I suspect they will get 
removed for this, but that's not my choice to make.)