Bug#1053700: xarray test failure on s390x

[Rebecca N. Palmer]

Control: tags -1 patch

These all come from xarray/tests/test_coding_times.py, and are probably 
fixable by replacing both instances of ("<M8[ns]") by (">M8[ns]" if 
sys.byteorder=='big' else "<M8[ns]") (remember to import sys), but I 
haven't actually tried that.