metrics/histograms: 43%/20s faster presubmits by using python 3

Specifically, CheckChangeOnCommit went from 47 seconds to 27 seconds on
a P920.

Using Python3 gives the presubmit checks a nice speed boost, and python2
is deprecated, so switch to 3. Note that USE_PYTHON3 in the
PRESUBMIT.py only affects the PRESUBMIT.py file itself, so we still need
to run the children with python3 explicitly. We could also run them
in-process instead of spawning child processes, I'm not sure why it was
done this way so I'm keeping it the same.

Bug: 1294667
Test: Run with/without change
Change-Id: I3c1423ccd802c37f2170084e431e0d0ed2759c84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3445802


Auto-Submit: David Munro <davidmunro@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/main@{#968355}