Add type hints for all packages

Type hints allow us to perform static type checking, among other things. We could benefit a lot from using them.

This stackoverflow answer does a great job at describing their main benefits.

pyannotate would come in handy if it had support for py3-style annotations, but meanwhile we can add them manually

monkeytype may actually be more useful than pyannotate as it generates py3-style annotations

Definition of done

• type hints for all of evm, p2p, and trinity modules.
• individual mypy CI runs for each of these modules using mypy --follow-imports=silent --ignore-missing-imports --check-untyped-defs --disallow-incomplete-defs -p <module>

This should not be done as a single pull request, but rather many small ones to iteratively add type hints, slowly expanding the mypy coverage as hints are added.

[cburgdorf]

@pipermerriam I claimed that bounty (or better signaled that I’m working on it) since I already started working on this (https://github.com/ethereum/py-evm/commit/57c6258a69662ee850e83c32fdc039e7fc936e87, https://github.com/ethereum/py-evm/commit/ed05aadfe1ed2196be514a666298cb58ff53c19d, https://github.com/ethereum/py-evm/commit/fb87f47bc7e8550901fa639740dea78676163973) and plan to continue to work on it.

[pipermerriam]

I don’t want to take this away from anyone (so speak up if you happen to be halfway through this or something). I am going to close this. We’ll still be adding type hints, but it will now be something iterative after some of the incoming refactors happen.