Test inline mypy configuration with more stable mypy option

The mypy configuration --no-strict-optional is discouraged and not much
tested. It caused the test to fail with >=mypy-1.6.0.

Closes #137

pytest_mypy_plugins/tests/test-mypy-config.yml

@@ -1,9 +1,9 @@
 # Also used in `test_explicit_configs.py`
 
-- case: custom_mypy_config_strict_optional_true_set
+- case: custom_mypy_config_disallow_any_explicit_set
+  expect_fail: yes
   main: |
-    from typing import Optional
-    a: Optional[int] = None
-    a + 1  # should not raise an error
+    from typing import Any
+    a: Any = None  # should raise an error
   mypy_config: |
-    strict_optional = false
+    disallow_any_explicit = true