Type hinting setup overhaul

[Kircheneer]

Currently mypy uses an implicit configuration. We should do the followings things:

• In what ways do we want to deviate from the default config?
• Add a tool.mypy section to pyproject.toml
• Make sure the configuration is used in the CI
• Make sure we support Python <3.9 by using List, Dict etc. from typing rather than list
• Drop all type hints from doc strings