Fix eth-account docs and core tests

[kclowes]

What was wrong/how was it fixed?

• the core tests were still using the deprecated signHash instead of the new unsafe_sign_hash
• the doctests break between eth-account 0.12.2, and eth-account 0.13.0, so require eth-account 0.13.x in docs dependencies. web3py should work fine with both, just emit warnings.

Todo:

• [x] Clean up commit history
• [x] Add or update documentation related to these changes
• [x] Add entry to the release notes

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