Fix bug because of str different meaning in py2 and py3

Author: Adrien Chauve

mrq/queue.py

@@ -13,7 +13,7 @@
 
 import sys
 PY3 = sys.version_info > (3,)
-from builtins import bytes
+from builtins import bytes, str
 from future import standard_library
 standard_library.install_aliases()
 
@@ -161,7 +161,7 @@ def serialize_job_ids(self, job_ids):
         elif isinstance(job_ids[0], ObjectId):
             return [x.binary for x in job_ids]
         else:
-            return [bytes.fromhex(str(x)) for x in job_ids]
+            return [bytes.fromhex(bytes(x)) for x in job_ids]
 
     def unserialize_job_ids(self, job_ids):
         """ Unserialize job_ids stored in Redis """