api: require python3 in more modules

These are all used by python3-only code, so mark them as such to
avoid possible regressions.

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

Change-Id: Ie68bca9dae9cf5dd213d3857d1856ee8895605a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2154061
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/api/controller/__init__.py b/api/controller/__init__.py
index 0bfe831..ab4cd48 100644
--- a/api/controller/__init__.py
+++ b/api/controller/__init__.py
@@ -3,6 +3,14 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+from __future__ import print_function
+
+import sys
+
+
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 IMPORT_PATTERN = 'chromite.api.controller.%s'
 
 # Endpoint ran successfully.