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.py

@@ -33,7 +33,8 @@ __all__ = [
 # See the EBNF at the top of the file to understand the logical connection
 # between the various node types.
 
-builtin_types = {'identifier', 'string', 'bytes', 'int', 'object', 'singleton'}
+builtin_types = {'identifier', 'string', 'bytes', 'int', 'object', 'singleton',
+                 'constant'}
 
 class AST:
     def __repr__(self):