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

Python/symtable.c

@@ -1455,6 +1455,7 @@ symtable_visit_expr(struct symtable *st, expr_ty e)
     case JoinedStr_kind:
         VISIT_SEQ(st, expr, e->v.JoinedStr.values);
         break;
+    case Constant_kind:
     case Num_kind:
     case Str_kind:
     case Bytes_kind: