[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>
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 43675cc..397df6e 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -1,3 +1,4 @@
+#!/usr/bin/env python3
 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
@@ -9,8 +10,6 @@
 for more details about the presubmit API built into gcl.
 """
 
-import collections
-import csv
 import fnmatch
 import os
 import re