Don't re-ping reviewers who have already approved.

[ammaraskar]

The bot continues to ping everyone who has submitted a review, even if they have approved their review. See this example where Serhiy was pinged. python/cpython#3066 (comment)

[brettcannon]

It's on purpose because if you changed the code in a later review that new code may need another round of reviews from previous reviewers.