`ast.AST._attributes` is used by `ast.dump()` but not documented

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-11-11.14:31:03.695>
labels = []
title = '`ast.AST._attributes` is used by `ast.dump()` but not documented'
updated_at = <Date 2019-11-21.22:23:42.058>
user = 'https://bugs.python.org/pekkaklarck'

bugs.python.org fields:

activity = <Date 2019-11-21.22:23:42.058>
actor = 'pekka.klarck'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2019-11-11.14:31:03.695>
creator = 'pekka.klarck'
dependencies = []
files = []
hgrepos = []
issue_num = 38765
keywords = []
message_count = 7.0
messages = ['356362', '356363', '356713', '356738', '357027', '357086', '357226']
nosy_count = 5.0
nosy_names = ['brett.cannon', 'terry.reedy', 'benjamin.peterson', 'pekka.klarck', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue38765'
versions = []