Add PEP484 Annotations

[iamdefinitelyahuman]

The Issue

The solcx codebase doesn't make use of PEP484 annotations. Adding them will make the code more readable and help prevent future bugs from being introduced.

Running MyPy with the following configuration should complete without errors.

[mypy]
disallow_untyped_defs = True
ignore_missing_imports = True
follow_imports = silent
files = solc/