Hack around #3440 (#3442)

Author: crusaderky

ci/azure/install.yml

@@ -16,7 +16,7 @@ steps:
         --pre \
         --upgrade \
         matplotlib \
-        pandas \
+        pandas=0.26.0.dev0+628.g03c1a3db2 \  # FIXME https://github.com/pydata/xarray/issues/3440
         scipy
         # numpy \  # FIXME https://github.com/pydata/xarray/issues/3409
     pip install \