service: convert most modules to python3

We can't convert the artifacts module as it's tangled up in cbuildbot.
But all the rest should be fine to migrate now.

BUG=chromium:997354
TEST=CQ passes

Change-Id: I119bdc51468d16c812562575892cc40e9ee91b2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2081373
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/service/packages_unittest.py b/service/packages_unittest.py
index ad4edcc..f90df38 100644
--- a/service/packages_unittest.py
+++ b/service/packages_unittest.py
@@ -10,6 +10,7 @@
 import json
 import os
 import re
+import sys
 
 from google.protobuf import json_format
 from google.protobuf.field_mask_pb2 import FieldMask
@@ -30,6 +31,10 @@
 from chromite.lib.uprev_lib import GitRef
 from chromite.service import packages
 
+
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 D = cros_test_lib.Directory