Update mypy Workaround if It Is No Longer Needed

# 💡 Summary #

We currently work around a perceived issue with type hinting with `mypy` in:
https://github.com/cisagov/hash-http-content/blob/36d4e52abd6e639343023ea09a6ee5c7a217690f/src/hash_http_content/hasher.py#L33-L44
After a new version of `mypy` with type hint updates (possibly v0.900) is released, we should see if this workaround is still necessary to pass linting.

## Motivation and context ##

Workarounds should only be used as long as there is something to work around.

## Implementation notes ##

The preferred usage is commented out, so testing is simple as switching between the current and preferred usage.

## Acceptance criteria ##

- [ ] `mypy` hook passes with the preferred usage in place
- [ ] Code is updated

or

- [ ] `mypy` hook still fails with the preferred usage in place
- [ ] Comment is updated to reflect the necessity of the workaround