Consider change the type annotation of active_chain to List[Block]

As Python typing document shows, An iterable type may not be proper for "__getitem__"" call, we may change it to List[Block] for better type checking.

https://docs.python.org/3/library/collections.abc.html

Search the repository for the active_chain annotation and inspect its use of __getitem__ and Block. Review the linked Python typing documentation, change the annotation to List[Block] if appropriate, and confirm the project’s type checking still accepts the updated declaration.

Written by the indexing model from the issue text.