autotest_quickmerge: fix under Python 3

UnboundMethodType as a concept no longer exists.  It's just a
function type now.  Switch the code to use callable instead.

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

Change-Id: I355adf02d8a6baa6c22cea2e0da048712f27c16b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1869502
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/autotest_quickmerge_unittest.py b/scripts/autotest_quickmerge_unittest.py
index 5d485a1..73697fa 100644
--- a/scripts/autotest_quickmerge_unittest.py
+++ b/scripts/autotest_quickmerge_unittest.py
@@ -7,8 +7,6 @@
 
 from __future__ import print_function
 
-import types
-
 import mock
 
 from chromite.lib import cros_test_lib
@@ -192,6 +190,6 @@
       self.fail('Required writeContentsToContentsFile function does '
                 'not exist.')
 
-    self.assertIsInstance(f, types.UnboundMethodType,
-                          'Required writeContentsToContentsFile is not '
-                          'a function.')
+    self.assertTrue(
+        callable(f),
+        msg='Required writeContentsToContentsFile is not a function.')