This pulls the latest version from upstream.

This drops the custom keys Chromium has been carrying.
We switched our release signing over to the official key,
so there's no need for ours anymore.

The new version will attempt to use Python 3 if available
and whine if it's not.  It shouldn't cause errors, just new
warnings, but it should help us track down old systems.

Bug: https://crbug.com/gerrit/10418
Test: `repo sync` still works
Change-Id: I915d0874a1642880831964f91c4632f9d17a1e4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2044341Reviewed-by: Michael Mortensen <mmortensen@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>