Revert "api: migrate to python3 only"

This reverts commit 2f29f97dd50fe325f2b86e9d0fda0723bfc87e0d.

Reason for revert: Python 3 failure in uprev android

Original change's description:
> api: migrate to python3 only
> 
> Convert all the API modules to require Python 3.6.  These only get
> imported by two programs, and we've migrated those to Python 3.6.
> We omit chromite.api.gen for now because that is widely imported
> by other programs that still support Python 2.
> 
> BUG=chromium:997354
> TEST=`./run_tests` passes
> 
> Change-Id: I35f64dc12923586817fa5869faee60efdfd21726
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2052466
> Reviewed-by: Alex Klein <saklein@chromium.org>
> Tested-by: Mike Frysinger <vapier@chromium.org>
> Commit-Queue: Mike Frysinger <vapier@chromium.org>

BUG=chromium:997354, chromium:1052524

Change-Id: I70056bc2de915630e21716515a4aed942709bd5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2056606
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Alex Klein <saklein@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
diff --git a/api/controller/test_unittest.py b/api/controller/test_unittest.py
index b0a36dc..156a3d2 100644
--- a/api/controller/test_unittest.py
+++ b/api/controller/test_unittest.py
@@ -7,10 +7,9 @@
 
 from __future__ import print_function
 
-import contextlib
 import os
-import sys
 
+import contextlib
 import mock
 
 from chromite.api import api_config
@@ -29,9 +28,6 @@
 from chromite.utils import key_value_store
 
 
-assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
-
-
 class DebugInfoTestTest(cros_test_lib.MockTempDirTestCase,
                         api_config.ApiConfigMixin):
   """Tests for the DebugInfoTest function."""