Update commit_id.py to use python3. This is blocking ANGLE rolls into WebKit. macOS no longer ships a "python" binary, only "python3". Bug: angleproject:7115 Change-Id: Ieb9473f393d006d64c18167bada2fe76ce996bb4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3537512 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>