Web3 tooling has broken type-checking

## What was wrong?
The `Discovery5Module` in the web3 tooling is not being type-checked correctly. The mypy check passes successfully when the attached `Method`s are missing a type and when the declared type is incorrect.

### Code that produced the error
https://github.com/ethereum/ddht/blob/master/ddht/tools/web3.py#L286-L355

### Expected Result
Mypy runs should fail when attached methods are missing a type and if the declared type is incorrect.

## How can it be fixed?
From https://github.com/ethereum/ddht/pull/224#discussion_r528919231

> My guess is that web3.py might be leaking Any which is typically where this type of deficiency shows up.

Start with ddht/tools/web3.py, especially lines 286-355, and inspect how Discovery5Module exposes its attached Methods to mypy. Run the existing mypy checks with a missing type and an incorrect declared type to reproduce the gap. Done means both cases make mypy fail instead of passing.

Written by the indexing model from the issue text.