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.

Include/internal/pycore_global_objects.h

@@ -66,9 +66,6 @@ struct _Py_static_objects {
 struct _Py_interp_cached_objects {
     PyObject *interned_strings;
 
-    /* AST */
-    PyObject *str_replace_inf;
-
     /* object.__reduce__ */
     PyObject *objreduce;
     PyObject *type_slots_pname;