## Description:
Make typing checks more strict: add `--strict-optional` flag that forbids implicit None return type. This flag will become default in the next version of mypy (0.600)
Add `homeassistant/util/` to checked dirs.
## Checklist:
- [x] The code change is tested and works locally.
- [x] Local tests pass with `tox`. **Your PR cannot be merged unless tests pass**
Andrey
Ville Skyttä
Fabian Affolter
Paulus Schoutsen