don't remember if there's a magic comment that contributors can
leave to get the bot to check again on its own, but the usual trigger
for this is the submitter posting to say they've signed it, a core dev
or triager seeing that and removing the "CLA Not Signed" label,

There is no magic comment, and to reduce work from core dev/triager, contributors can use this to update their own PRs https://check-python-cla.herokuapp.com/

Being able to update the CLA as soon as it gets signed instead of waiting a couple days, is on top of my wishlist:

https://github.com/python/core-workflow/issues/360