Cannot parse block_info transactions into SignedTransaction objects

### Subject of the issue

`SignedTransaction.undictify` does not work on block_info transactions from `block_info().block.txns`

### Steps to reproduce

1. Fetch `block_info` using Algod client
2. Try to parse transaction for transactions using `SignedTransaction.undictify`

### Expected behaviour

Transactions get parsed into objects. Potential solution could be to have an additional method that parses the entire block into readable data.

### Actual behaviour

Bunch of errors where undictify assumed keys are bytes and not base64 encoded strings