Add more answer_dist coverage.

Also resolve remaining PEP8 warnings.

Change-Id: I1e5e1947ededbba97540e661c3fb6adcac100597

edx/analytics/tasks/reports/incremental_enrollments.py

@@ -293,7 +293,7 @@ class DailyRegistrationsEnrollmentsAndCourses(luigi.Task,
         """
 
         report = pandas.DataFrame.from_items([
-            (self.ROW_LABELS['registrations'],  registrations),
+            (self.ROW_LABELS['registrations'], registrations),
             (self.ROW_LABELS['enrollments'], enrollments),
         ])
         return report

edx/analytics/tasks/reports/tests/test_incremental_enrollments.py

@@ -277,8 +277,6 @@ class TestWeeklyIncrementalUsersAndEnrollments(unittest.TestCase):
         self.assertEqual(res.loc[self.row_label('enrollment_change')]['2013-01-15'], 2)
 
 
-
-
 class TestDailyRegistrationsEnrollmentsAndCourses(unittest.TestCase):
     """Tests for DailyRegistrationsEnrollmentsAndCourses class."""