Static type checking in the CI

Python supports type annotations since Python 3.6, but Octavia code hasn't used it much so far. By adding more type annotations over time and using a type checker like mypy, we could enable type checks that can be run using tox locally and in the CI system.

Type checking can help detect certain bugs and improve the overall code quality. Apart from that, type annotations can be seen as part of the documentation of the code and help developers and IDE understand the code better.