Fix for bug with in operation on optionals in no-strict-optional mode (#14727)

Fixes a bug introduced in #14384
wherein a union that includes `None` is no longer treated as a valid
right-hand type for the `in` operator in `no-strict-optional` mode. (The
reported error is `error: "None" has no attribute "__iter__" (not
iterable) [attr-defined]`)

Author: koogoro

Diff for: mypy/checkexpr.py

@@ -2950,7 +2950,7 @@ def visit_comparison_expr(self, e: ComparisonExpr) -> Type:
                 right_type = get_proper_type(right_type)
                 item_types: Sequence[Type] = [right_type]
                 if isinstance(right_type, UnionType):
-                    item_types = list(right_type.items)
+                    item_types = list(right_type.relevant_items())
 
                 sub_result = self.bool_type()
 

Diff for: test-data/unit/check-optional.test

@@ -1031,3 +1031,12 @@ def f1(b: bool) -> Optional[int]:
 class Defer:
     def __init__(self) -> None:
         self.defer = 10
+
+[case testOptionalIterator]
+# mypy: no-strict-optional
+from typing import Optional, List
+
+x: Optional[List[int]]
+if 3 in x:
+    pass
+