Use format specification mini-language to format string (#1558)

Co-authored-by: Joe Hamman <joe@earthmover.io>

zarr/storage.py

@@ -784,7 +784,7 @@ def __len__(self):
         return len(self._mutable_mapping)
 
     def __repr__(self):
-        return f"<{self.__class__.__name__}: \n{repr(self._mutable_mapping)}\n at {hex(id(self))}>"
+        return f"<{self.__class__.__name__}: \n{self._mutable_mapping!r}\n at {id(self):#x}>"
 
     def __eq__(self, other):
         if isinstance(other, KVStore):