Fix compatibility regression in inspector output module. (#706)

* Fix compatibility regression in inspector output module.

* Explicit version-specific branching.

* Better Python3 compatibility approach.

* Remove extraneous sys import.

openhtf/output/callbacks/mfg_inspector.py

@@ -33,6 +33,10 @@
 import os
 import threading
 import zlib
+try:
+  from past.types import unicode
+except ImportError:
+  pass
 
 import httplib2
 import oauth2client.client
@@ -155,8 +159,8 @@ def _extract_attachments(phase, testrun, used_parameter_names):
     name = _ensure_unique_parameter_name(name, used_parameter_names)
     testrun_param = testrun.info_parameters.add()
     testrun_param.name = name
-    if isinstance(attachment_data, str):
-      attachment_data = attachment_data.encode('utf8')
+    if isinstance(attachment_data, unicode):
+      attachment_data = attachment_data.encode('utf-8')
     testrun_param.value_binary = attachment_data
     if mimetype in MIMETYPE_MAP:
       testrun_param.type = MIMETYPE_MAP[mimetype]