Consider change the type annotation of utxo_set to MutableMapping[OutPoint, UnspentTxOut]

As Python typing document shows, A Mapping type may not be proper for "setitem"" call, we may change it to MutableMapping for better type checking.

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

Start by locating the utxo_set type annotation and review the linked Python collections.abc documentation about Mapping and MutableMapping. Done means the annotation permits setitem operations and the project's type checking passes.

Written by the indexing model from the issue text.