Fix mypy complaint

Author: waketzheng

tortoise/fields/data.py

@@ -419,7 +419,7 @@ def to_db_value(
     ) -> DateFieldQueryValueType | None:
         if value is not None and isinstance(value, str) and len(value) > 4:
             with contextlib.suppress(ValueError):
-                value = parse_datetime(value).date()
+                value = parse_datetime(value).date()  # type: ignore[assignment]
         self.validate(value)
         return value