Use mypy annotations

[bergercookie]

Since colcon packages mandate Pytyhon 3.5 or higher, we can use explicit mypy annotations to document function parameters and output types instead of docstrings. I'm also removing the docstrings for the types since sphinx can parse and create documentation for mypy types.

Current PR also includes: Fix KeyError exception when dependencies Cargo key is not found (it's tightly coupled with the change in mypy types - hence I can't make a separate PR for it.