my_activity.py: Run using vpython3 by default.

Also, prompt users to use vpython3 when running my_activity.py
using Python 2.

Change-Id: I078517832520b3c7baf3324f83e9a3b5194a5bcb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2079703
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

my_activity.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env vpython
+#!/usr/bin/env vpython3
 # Copyright (c) 2012 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.
@@ -46,6 +46,8 @@ import gerrit_util
 
 
 if sys.version_info.major == 2:
+  logging.warning(
+      'Python 2 is deprecated. Run my_activity.py using vpython3.')
   import urllib as urllib_parse
 else:
   import urllib.parse as urllib_parse