Bug#1066801: pandas: FTBFS: /usr/lib/python3/dist-packages/dateutil/zoneinfo/__init__.py:26: UserWarning: I/O error(2): No such file or directory

[Rebecca N. Palmer]

Control: retitle -1 pandas: test-failing warning with new xarray

This is a warning being treated as an error, but the one in 
test_to_xarray (probably due to the new version of xarray), not the 
zoneinfo one.  This is a FutureWarning, so it should be OK to *use* the 
current pandas with the new xarray.

The same warning happens in autopkgtest but isn't treated as an error 
there, so didn't block the new xarray.

I intend to try fixing this later.