upload_prebuilts: migrate to python3 only

Now that cbuildbot is python3 only, we can migrate this module.

BUG=chromium:997354
TEST=`./run_tests` passes

Change-Id: I28829945ac5cdbeb4f93f2b4e15a07588ec55dfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2154062
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/upload_prebuilts_unittest.py b/scripts/upload_prebuilts_unittest.py
index e09afec..7bea10d 100644
--- a/scripts/upload_prebuilts_unittest.py
+++ b/scripts/upload_prebuilts_unittest.py
@@ -10,6 +10,7 @@
 import copy
 import os
 import multiprocessing
+import sys
 import tempfile
 
 import mock
@@ -26,6 +27,9 @@
 # pylint: disable=protected-access
 
 
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 PUBLIC_PACKAGES = [{'CPV': 'gtk+/public1', 'SHA1': '1', 'MTIME': '1'},
                    {'CPV': 'gtk+/public2', 'SHA1': '2',
                     'PATH': 'gtk+/foo.tgz', 'MTIME': '2'}]