feat: add support for EIP-712 message signing

[lost-theory]

This change adds a dependency for the eip712 package and uses it to implement a new LocalAccount.sign_message method.

What I did

Related issue: ApeWorX/eip712#4

How I did it

Added the eip712 dependency to requirements.in, used pip-compile to update requirements.txt, used that new package to implement the LocalAccount.sign_message method, wrote a new test that defines an EIP712Message and signs it using a local account with a private key loaded, verified the test result, ran lint & other pre-commit steps, added docs, added entry in the changelog.

How to verify it

I wrote a new test alongside the existing LocalAccount tests in test_accounts.py

$ pytest -k test_sign_message 
...
== 1 passed in 12.87s ==

Checklist

• I have confirmed that my PR passes all linting checks
• I have included test cases
• I have updated the documentation
• I have added an entry to the changelog

[BlinkyStitt]

This looks great! if I could merge, I would.

[skellet0r]

@lost-theory pls rebase 🙏

[lost-theory]

Rebased @skellet0r.