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**

tests/util/test_yaml.py

@@ -411,6 +411,22 @@ class TestSecrets(unittest.TestCase):
         assert mock_error.call_count == 1, \
             "Expected an error about logger: value"
 
+    def test_secrets_are_not_dict(self):
+        """Did secrets handle non-dict file."""
+        FILES[self._secret_path] = (
+                  '- http_pw: pwhttp\n'
+                  '  comp1_un: un1\n'
+                  '  comp1_pw: pw1\n')
+        yaml.clear_secret_cache()
+        with self.assertRaises(HomeAssistantError):
+            load_yaml(self._yaml_path,
+                      'http:\n'
+                      '  api_password: !secret http_pw\n'
+                      'component:\n'
+                      '  username: !secret comp1_un\n'
+                      '  password: !secret comp1_pw\n'
+                      '')
+
 
 def test_representing_yaml_loaded_data():
     """Test we can represent YAML loaded data."""