[3.12] Clarify one-item tuple (GH-114745) (#114757)

A 'single tuple' means 'one tuple, of whatever length.
Remove the unneeded and slightly distracting parenthetical 'singleton' comment.
(cherry picked from commit a1332a99cf1eb9b879d4b1f28761b096b5749a0d)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>