build: update shebang to python3

This assumes that CQ has enough coverage for these scripts if they are
important.

Derived from https://crrev.com/c/4078643/4/build/android/pylib/symbols/mock_addr2line/mock_addr2line#1

This is made by
$ rg -l '#!.*python$' build | \
  xargs sed -i 's/\(#!.*\)python$/\1python3/'

Bug: 1406153
Change-Id: I3df47e000c690ce8af341ff317539c4faa58e3bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4159758
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1091953}

diff --git a/build/get_symlink_targets.py b/build/get_symlink_targets.py
index 33362c0..850bbae 100755
--- a/build/get_symlink_targets.py
+++ b/build/get_symlink_targets.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright 2019 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.