[test_unicode_literals] Arm unicode_literals check

From now on, the line

from __future__ import unicode_literals

should be contained in every single Python file lest we run into any more 2.x/3.x issues.
Going forward, we're likely to develop on 3.x only and would likely miss subtle bugs otherwise.

youtube_dl/postprocessor/__init__.py

@@ -1,3 +1,4 @@
+from __future__ import unicode_literals
 
 from .atomicparsley import AtomicParsleyPP
 from .ffmpeg import (

youtube_dl/postprocessor/common.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 from ..utils import PostProcessingError
 
 

youtube_dl/postprocessor/xattrpp.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 import os
 import subprocess
 import sys