Add ast.Constant

Issue #26146: Add a new kind of AST node: ast.Constant. It can be used by
external AST optimizers, but the compiler does not emit directly such node.

An optimizer can replace the following AST nodes with ast.Constant:

* ast.NameConstant: None, False, True
* ast.Num: int, float, complex
* ast.Str: str
* ast.Bytes: bytes
* ast.Tuple if items are constants too: tuple
* frozenset

Update code to accept ast.Constant instead of ast.Num and/or ast.Str:

* compiler
* docstrings
* ast.literal_eval()
* Tools/parser/unparse.py

Parser/asdl_c.py

@@ -834,6 +834,7 @@ static PyObject* ast2obj_object(void *o)
     return (PyObject*)o;
 }
 #define ast2obj_singleton ast2obj_object
+#define ast2obj_constant ast2obj_object
 #define ast2obj_identifier ast2obj_object
 #define ast2obj_string ast2obj_object
 #define ast2obj_bytes ast2obj_object
@@ -871,6 +872,26 @@ static int obj2ast_object(PyObject* obj, PyObject** out, PyArena* arena)
     return 0;
 }
 
+static int obj2ast_constant(PyObject* obj, PyObject** out, PyArena* arena)
+{
+    if (obj == Py_None || obj == Py_True || obj == Py_False) {
+        /* don't increment the reference counter, Constant uses a borrowed
+         * reference, not a strong reference */
+        *out = obj;
+        return 0;
+    }
+
+    if (obj) {
+        if (PyArena_AddPyObject(arena, obj) < 0) {
+            *out = NULL;
+            return -1;
+        }
+        Py_INCREF(obj);
+    }
+    *out = obj;
+    return 0;
+}
+
 static int obj2ast_identifier(PyObject* obj, PyObject** out, PyArena* arena)
 {
     if (!PyUnicode_CheckExact(obj) && obj != Py_None) {