[3.13] gh-122300: Preserve AST nodes for format specifiers with single elements (GH-122308) (#122364)

(cherry picked from commit db2d8b6)

Author: pablogsal

Doc/library/ast.rst

@@ -316,7 +316,9 @@ Literals
                             args=[
                                 Name(id='a', ctx=Load())]),
                         conversion=-1,
-                        format_spec=Constant(value='.3'))]))
+                        format_spec=JoinedStr(
+                            values=[
+                                Constant(value='.3')]))]))
 
 
 .. class:: List(elts, ctx)