export_olx management command: write bytes to stdout correctly

This must have been broken ever since we upgraded from Python 2 to 3.

DENG-379

cms/djangoapps/contentstore/management/commands/export_olx.py

@@ -60,13 +60,19 @@ class Command(BaseCommand):
 
         export_course_to_tarfile(course_key, filename)
 
-        results = self._get_results(filename) if pipe_results else ''
+        results = self._get_results(filename) if pipe_results else b''
 
-        self.stdout.write(results, ending="")
+        # results is of type bytes, so we must write the underlying buffer directly.
+        self.stdout.buffer.write(results)
 
     def _get_results(self, filename):
-        """Load results from file"""
-        with open(filename, 'rb') as f:  # pylint: disable=open-builtin
+        """
+        Load results from file.
+
+        Returns:
+            bytes: bytestring of file contents.
+        """
+        with open(filename, 'rb') as f:
             results = f.read()
             os.remove(filename)
         return results