Bug#1043240: transition: pandas 1.5 -> 2.0/2.1

[Rebecca N. Palmer]

dask has now been updated to a pandas 2 compatible version, allowing me 
to test its reverse dependencies.  This found that intake and 
xarray-sentinel are broken by pandas 2, while python-xarray is already 
broken for other reasons.

A common source of failures is that pandas.util.testing has been renamed 
to pandas.testing.  Both names were available in all 1.x versions (and 
hence in Debian stable and oldstable), so Debian packages that were 
using this can immediately switch unconditionally.

pandas 2.1 is likely to be released soon.  If this happens before we are 
ready to move 2.x to unstable, I plan to go to 2.1 in one transition.