tests: unconditionalize _unified_diff

Now that we're Python 3 only we can make this logic simpler.

Differential Revision: https://phab.mercurial-scm.org/D12237

tests/run-tests.py

@@ -52,6 +52,7 b' import contextlib'
 import difflib
 import distutils.version as version
 import errno
+import functools
 import json
 import multiprocessing
 import os
@@ -879,11 +880,7 b' def makecleanable(path):'
                 pass
 
 
-_unified_diff = difflib.unified_diff
-if PYTHON3:
-    import functools
-
-    _unified_diff = functools.partial(difflib.diff_bytes, difflib.unified_diff)
+_unified_diff = functools.partial(difflib.diff_bytes, difflib.unified_diff)
 
 
 def getdiff(expected, output, ref, err):