Add `hexbytes` upper pin for web3.py `v6`

### What was wrong?

Some breaking changes are coming to `hexbytes`, along with a new "major" version. Let's figure out what version that will be and add an upper pin to web3.py `v6` for `< {new hexbytes version}` in *setup.py*.

This new version will be used in the `v7` branch as a lower bound.