Enable warn_unreachable for mypy self-check

mypy/nodes.py

@@ -176,11 +176,11 @@ class Node(Context):
     __slots__ = ()
 
     def __str__(self) -> str:
-        ans = self.accept(mypy.strconv.StrConv(options=Options()))
-        if ans is None:
+        a = self.accept(mypy.strconv.StrConv(options=Options()))
+        if a is None:
             # Some visitors might have empty bodies and actually return `None`
             return repr(self)  # type: ignore[unreachable]
-        return ans
+        return a
 
     def str_with_options(self, options: Options) -> str:
         a = self.accept(mypy.strconv.StrConv(options=options))