[3.10] gh-125529: Avoid f-strings in the metagrammar (#125582)

Grammar actions need to be valid Python tokens and the accepted tokens need to be
listed in the actions mini-grammar).

In Python 3.12+ (PEP 701), f-strings are no longer STRING tokens, so pegen fails
to regenerate the metaparser on this Python version, as in:

   PYTHON_FOR_REGEN=python3.12 make regen-pegen-metaparser

Use `+` and plain strings rather than f-strings.

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

Author: encukou

Tools/peg_generator/pegen/grammar_parser.py

@@ -420,7 +420,7 @@ def named_item(self) -> Optional[NamedItem]:
             and
             (item := self.item())
         ):
-            return NamedItem ( name . string , item , f"{type.string}*" )
+            return NamedItem ( name . string , item , type . string + "*" )
         self.reset(mark)
         if cut: return None
         cut = False

Tools/peg_generator/pegen/metagrammar.gram

@@ -84,7 +84,7 @@ items[NamedItemList]:
     | named_item { [named_item] }
 
 named_item[NamedItem]:
-    | NAME '[' type=NAME '*' ']' '=' ~ item {NamedItem(name.string, item, f"{type.string}*")}
+    | NAME '[' type=NAME '*' ']' '=' ~ item {NamedItem(name.string, item, type.string+"*")}
     | NAME '[' type=NAME ']' '=' ~ item {NamedItem(name.string, item, type.string)}
     | NAME '=' ~ item {NamedItem(name.string, item)}
     | item {NamedItem(None, item)}