Update zarr to 2.8.1

>=2.8.0 required for dimension_separator support: zarr-developers/zarr-python#716

Author: melissalinkert

setup.py

@@ -82,7 +82,7 @@ def read(fname):
           'click==7.0',
           'pillow>=7.1.0',
           'numpy==1.17.3',
-          'zarr==2.4.0',
+          'zarr==2.8.1',
           'kajiki==0.8.2',
       ],
       tests_require=[