Bug#1043240: transition: pandas 1.5 -> 2.1

[Rebecca N. Palmer]

Remaining blockers for testing migration:
- python-ulmo #1044057: has a patch, please upload
- pydevd #1063274: unclear whether my patch breaks something else, 
please leave alone for now

Status unclear:
- python-xarray: autopkgtest has failed 3 times, but all 3 are 
(different) failures that have occurred without pandas 2, apparently at 
random
- q2-*: autopkgtest fails in "fixed" versions, but possibly because 
they're not all being tested together