Make presubmit_unittest py3 compatible

And run it in presubmit checks

R=apolito@google.com

Change-Id: I1521fe4dd9fd5fe391459a00a91f03399b15b36a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3292211
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Anthony Polito <apolito@google.com>

PRESUBMIT.py

@@ -110,7 +110,6 @@ def CheckUnitTestsOnCommit(input_api, output_api):
     ])
   py2_only_tests = [
       'fix_encoding_test.py',
-      'presubmit_unittest.py',
       'recipes_test.py',
   ]