feat: bytes32 validation with pydantic models

What I did

fixes: #20

First stab at this, it's a little more complicated by the pydantic validation to use it in BaseModel

When you write the validation method on the Bytes32 class it gets called in the BaseModel where it's being used, so the cls is the implementing class, not Bytes32, so you don't have access to the bytes_length. I'm open to alternative ways of making this a more flexible class, i.e. allowing for passing the number of expected bytes. All I ask is that we call the class ByteSized if we're able to get this dynamic validation working.

As for the JSON validation, it works well and we can pass the serialization for it in the BaseModel Config class by using the json_encoders dictionary and pointing it to the __serialize__ method in Bytes32. The issue arises when you want the dict() to also serialize differently, pydantic doesn't currently support this behavior but it's being talked about.

https://github.com/samuelcolvin/pydantic/issues/1409 https://github.com/samuelcolvin/pydantic/issues/951 https://github.com/samuelcolvin/pydantic/issues/811

How I did it

How to verify it

Checklist

[ ] Passes all linting checks (pre-commit and CI jobs)
[ ] New test cases have been added and are passing
[ ] Documentation has been updated
[ ] PR title follows Conventional Commit standard (will be automatically included in the changelog)

ApeWorX / ethpm-types