[infra] Update PRESUBMIT.py to run under Python3

No real changes, unlike https://skia-review.googlesource.com/c/buildbot/+/439856/

Change-Id: I9b83cc0d1ee207fad0747c663469b4bae61b2049
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/439780
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
1 file changed