Fix ast warnings for Python3.12 (#15558)

Warnings before:
#15330 (comment)
Warnings after:

```
(.venv312) ~/Desktop/mypy  master ✗                                                       
» python -m mypy ex.py
Success: no issues found in 1 source file
```

Zero :)

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

mypy/fastparse.py

@@ -1873,15 +1873,14 @@ def visit_BinOp(self, n: ast3.BinOp) -> Type:
             uses_pep604_syntax=True,
         )
 
-    # Only for 3.8 and newer
     def visit_Constant(self, n: Constant) -> Type:
         val = n.value
         if val is None:
             # None is a type.
             return UnboundType("None", line=self.line)
         if isinstance(val, str):
             # Parse forward reference.
-            return parse_type_string(n.s, "builtins.str", self.line, n.col_offset)
+            return parse_type_string(val, "builtins.str", self.line, n.col_offset)
         if val is Ellipsis:
             # '...' is valid in some types.
             return EllipsisType(line=self.line)