sync: update event is_set API

Python 3 renamed this method from isSet to is_set.

Change-Id: I8f9bb0b302d55873bed3cb20f2d994fa2d082157
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297742


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>