Here is a (perhaps incomplete) list of documented names absent in __all__ lists of respective modules. Perhaps some of them (but not all) are worth to be added to __all__ lists.

calendar.Calendar
calendar.HTMLCalendar
calendar.TextCalendar
cgi.test
configparser.Error
csv.unix_dialect
doctest.DocFileCase
doctest.DocTestCase
enum.EnumMeta
fileinput.fileno
ftplib.Error
ftplib.error_perm
ftplib.error_reply
gettext.bind_textdomain_codeset
gettext.lgettext
gettext.lngettext
http.client.HTTPMessage
http.cookies.Morsel
http.server.test
logging.shutdown
mailbox.Error
mailbox.ExternalClashError
mailbox.NoSuchMailboxError
mailbox.NotEmptyError
mimetypes.MimeTypes
optparse.check_choice
pickletools.OpcodeInfo
plistlib.InvalidFileException
pydoc.doc
smtpd.SMTPChannel
subprocess.SubprocessError
subprocess.TimeoutExpired
tarfile.CompressionError
tarfile.HeaderError
tarfile.ReadError
tarfile.open
threading.BrokenBarrierError
tkinter.ttk.Widget
tokenize.open
traceback.FrameSummary
traceback.StackSummary
traceback.TracebackException
traceback.walk_stack
traceback.walk_tb
wave.Wave_read
wave.Wave_write
xml.etree.ElementTree.XMLPullParser