Use pendulum to traverse mock reporting dates

Previously, subtracting 29 days could result in the same month listed
twice (for example, subtracting 29 days from Nov 30, would result in two
Novembers listed).

atst/domain/csp/reports.py

@@ -1,4 +1,3 @@
-import datetime
 from itertools import groupby
 from collections import OrderedDict
 import pendulum
@@ -33,10 +32,10 @@ class MockApplication:
 
 
 def generate_sample_dates(_max=8):
-    current = datetime.datetime.today()
+    current = pendulum.now()
     sample_dates = []
     for _i in range(_max):
-        current = current - datetime.timedelta(days=29)
+        current = current.subtract(months=1)
         sample_dates.append(current.strftime("%m/%Y"))
 
     reversed(sample_dates)