gh-125268: Use static string for "1e309" in AST (#125272)

When formatting the AST as a string, infinite values are replaced by
1e309, which evaluates to infinity. The initialization of this string
replacement was not thread-safe in the free threading build.

Python/Python-ast.c

@@ -281,8 +281,6 @@ void _PyAST_Fini(PyInterpreterState *interp)
     Py_CLEAR(state->vararg);
     Py_CLEAR(state->withitem_type);
 
-    Py_CLEAR(_Py_INTERP_CACHED_OBJECT(interp, str_replace_inf));
-
     state->finalized = 1;
     state->once = (_PyOnceFlag){0};
 }