Closes#13998
/cc @notpeter would you mind giving this branch a go to see if this is
pleasant to use? This impl is not quite what VSC has, but I think it
feels okay?
In this PR, the sequence goes as follows:
1st keypress: "|"
2nd keypress: ""|
3rd keypress: """|"""
Release Notes:
- Improved handling of triple-quote strings in Python.