I found this in Notepad++ under Settings -> Preferences -> Auto-completion -> Auto-insert with separate checkboxes for (, {, [, ', ", html/xml tags, and up to 3 custom pairs.  Way too elaborate for IDLE.  The only question to me is whether to have just one option for all 5 openers or separate option to complete ([{ versus quotes.

Concerns: a) matching triple quotes is a bit baroque, though possible the most useful part of the feature; b) except for completing tripple quotes, no keystrokes are saved, and then only if one used End to skip three chars at once.  On the other hand, at least some people who have use the feather elsewhere consider it a net win.

Implementation notes: 1. IDLE editors uses two mechanisms to respond to keystrokes.  The delegator mechanism checks all keystrokes and responds to some.  IDLE otherwise binds keystrokes to event-handlers or to pseudoevent bound to event-handlers.  Perhaps these could be unified and made more efficient.

2. In the meanwhile... Menu items are bound to pseudoevents, along with corresponding hot heys.  This issue will *not* add a menu entry "Add matching closer".  So the openers can be bound directly to event handlers that look at the opener.