python: Improve handling of triple-quoted strings (#20664)

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.