Make packager_test.py work with python 3.5

Addresses binary/str conversion problems like
'can't use a string pattern on a bytes-like object'.

With this change, packager_test.py will work with any version of python
after python2.5.

Fixes #553.

Change-Id: I02066942e4bcdddc1db1daf761abab4cc46169a9

packager/app/test/packager_app.py

@@ -45,11 +45,11 @@ class PackagerApp(object):
   def DumpStreamInfo(self, stream):
     input_str = 'input=%s' % stream
     cmd = [self.packager_binary, input_str, '--dump_stream_info']
-    return subprocess.check_output(cmd, env=self.GetEnv())
+    return subprocess.check_output(cmd, env=self.GetEnv()).decode()
 
   def Version(self):
     return subprocess.check_output(
-        [self.packager_binary, '--version'], env=self.GetEnv())
+        [self.packager_binary, '--version'], env=self.GetEnv()).decode()
 
   def Package(self, streams, flags=None):
     """Executes packager command."""

packager/app/test/packager_test.py

@@ -173,7 +173,7 @@ def _UpdateMediaInfoPaths(media_info_filepath):
   #   after:  media_file_name: "bear-640x360-audio.mp4"
 
   with open(media_info_filepath, 'rb') as f:
-    content = f.read()
+    content = f.read().decode()
 
   regex = 'media_file_name: "(.*)"'
   for path in re.findall(regex, content):
@@ -181,7 +181,7 @@ def _UpdateMediaInfoPaths(media_info_filepath):
     content = content.replace(path, short_path)
 
   with open(media_info_filepath, 'wb') as f:
-    f.write(content)
+    f.write(content.encode())
 
 
 def _UpdateMpdTimes(mpd_filepath):
@@ -200,7 +200,7 @@ def _UpdateMpdTimes(mpd_filepath):
     return out
 
   with open(mpd_filepath, 'rb') as f:
-    content = f.read()
+    content = f.read().decode()
 
   content = _Replace(
       content,
@@ -213,7 +213,7 @@ def _UpdateMpdTimes(mpd_filepath):
       'publishTime="some_time"')
 
   with open(mpd_filepath, 'wb') as f:
-    f.write(content)
+    f.write(content.encode())
 
 
 def GetExtension(input_file_path, output_format):