Replace dict.iterkeys with six.iterkeys to make PY3 compatible

Python3 do not use dict.iterkeys, which would raise AttributeError:
'dict' object has no attribute 'iterkeys'.

Change-Id: I97e320eac9f2f0b2cb5cf34a1d3fc57e80e440ed
Closes-Bug: #1596124

tools/colorizer.py

@@ -42,6 +42,7 @@
 """Display a subunit stream through a colorized unittest test runner."""
 
 import heapq
+import six
 import subunit
 import sys
 import unittest
@@ -276,7 +277,7 @@ class SubunitTestResult(testtools.TestResult):
         self.stopTestRun()
 
     def stopTestRun(self):
-        for cls in list(self.results.iterkeys()):
+        for cls in list(six.iterkeys(self.results)):
             self.writeTestCase(cls)
         self.stream.writeln()
         self.writeSlowTests()