Fix blobs cast

Author: jtraglia

tests/core/pyspec/eth2spec/test/helpers/fork_choice.py

@@ -178,7 +178,7 @@ def add_block(spec,
 
     # Check blob_data
     if blob_data is not None:
-        blobs = spec.List[spec.Blob, spec.config.MAX_BLOB_COMMITMENTS_PER_BLOCK](blob_data.blobs)
+        blobs = spec.List[spec.Blob, spec.MAX_BLOB_COMMITMENTS_PER_BLOCK](blob_data.blobs)
         blobs_root = blobs.hash_tree_root()
         yield get_blobs_file_name(blobs_root=blobs_root), blobs