Incorrect type for hash_value arg in sign_hash function

The `hash_value` arg of the `sign_hash` in `pkcs1.py` currently is typed out as optional, but `hash_value` is required, lest an error occur when we reach the line:

```
clear_text = asn1_code + hash_value
```

Open pkcs1.py and inspect the sign_hash function, focusing on the hash_value annotation and the concatenation line identified in the issue. Confirm how the required argument is represented in the surrounding API and update the annotation so it matches the runtime requirement; done means hash_value is no longer typed as optional.

Written by the indexing model from the issue text.