Mark decoding as not covered by Python 2 tests

Author: jakirkham

zarr/core.py

@@ -1924,7 +1924,7 @@ def hexdigest(self, hashname="sha1"):
         checksum = binascii.hexlify(self.digest(hashname=hashname))
 
         # This is a bytes object on Python 3 and we want a str.
-        if type(checksum) is not str:
+        if type(checksum) is not str:  # pragma: py2 no cover
             checksum = checksum.decode('utf8')
 
         return checksum