Skip coverage of temp file cleanup

In the ideal case, this cleanup step never happens on CI as the file got
moved into place and so no longer exists at the old location. Given
this, we ignore coverage on this line.

Author: jakirkham

zarr/storage.py

@@ -778,7 +778,7 @@ def __setitem__(self, key, value):
 
         finally:
             # clean up if temp file still exists for whatever reason
-            if temp_path is not None and os.path.exists(temp_path):
+            if temp_path is not None and os.path.exists(temp_path):  # pragma: no cover
                 os.remove(temp_path)
 
     def __delitem__(self, key):