Bug#942106: python3.8 / pandas py2removal

["Rebecca N. Palmer" <rebecca_palmer@zoho.com>]

mdp isn't in testing either, but if you're using a policy of "no 
py2removals that break packages in testing", tnseq-transit (Depends: 
statsmodels) and possibly stimfit (Recommends: pandas) need to be done 
as well.  Those are both thought to need new upstream versions.

(patsy isn't a leaf package for py2removal, it's in a cycle with pandas 
and statsmodels, i.e. for a non-breaking removal they all have to go 
together.)