Use pendulum for datetime operations when possible

Currently, we use both Python's built-in datetime library and Pendulum
to do datetime operations. For the sake of consistency, we should try to
stick to one library for datetimes. We could have used either, but
Pendulum has a more ergonomic API, so I decided to go with it when
possible.

The places where were we didn't / couldn't replace datetime are:
- checking instances of datetimes. Pendulum's objects are subclasses of
  python native datetime objects, so it's still useful to import
  datetime in those cases of using is_instance()
- WTForms date validators expect datetime style string formats --
  Pendulum has its own format for formatting/ parsing strings. As such,
  our custom validator DateRange needs to use datetime.stptime() to
  account for this format.

tests/models/test_task_order.py

@@ -1,6 +1,6 @@
 from werkzeug.datastructures import FileStorage
 import pytest
-from datetime import date
+import pendulum
 from unittest.mock import patch, PropertyMock
 import pendulum
 
@@ -13,11 +13,11 @@ from tests.mocks import PDF_FILENAME
 
 
 def test_period_of_performance_is_first_to_last_clin():
-    start_date = date(2019, 6, 6)
-    end_date = date(2020, 6, 6)
+    start_date = pendulum.date(2019, 6, 6)
+    end_date = pendulum.date(2020, 6, 6)
 
-    intermediate_start_date = date(2019, 7, 1)
-    intermediate_end_date = date(2020, 3, 1)
+    intermediate_start_date = pendulum.date(2019, 7, 1)
+    intermediate_end_date = pendulum.date(2020, 3, 1)
 
     task_order = TaskOrderFactory.create(
         clins=[