home-assistant / home-assistant / 46362

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Make typing checks more strict (#14429)

## 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**

48 of 48 new or added lines in 12 files covered. (100.0%)

36573 of 38947 relevant lines covered (93.9%)

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