Python-modernize on edx-platform (336 of 380)

After executing python-modernize and isort, few changes
occurred in import sequence as well as unicode func is
replaced by six.text_type func.

INCR-439

lms/djangoapps/grades/admin.py

@@ -1,8 +1,11 @@
 """
 Django admin page for grades models
 """
+from __future__ import absolute_import
+
 from config_models.admin import ConfigurationModelAdmin, KeyedConfigurationModelAdmin
 from django.contrib import admin
+
 from lms.djangoapps.grades.config.forms import CoursePersistentGradesAdminForm
 from lms.djangoapps.grades.config.models import (
     ComputeGradesSetting,