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/routes/task_orders/test_index.py

@@ -1,7 +1,5 @@
-from datetime import date
 from flask import url_for
 import pytest
-from datetime import timedelta, date
 
 from atst.domain.permission_sets import PermissionSets
 from atst.domain.task_orders import TaskOrders

tests/routes/task_orders/test_new.py

@@ -1,6 +1,6 @@
 import pytest
 from flask import url_for, get_flashed_messages
-from datetime import timedelta, date
+import pendulum
 from uuid import uuid4
 
 from atst.domain.task_orders import TaskOrders
@@ -339,7 +339,7 @@ def test_task_orders_submit_task_order(client, user_session, task_order):
     )
     assert response.status_code == 302
 
-    active_start_date = date.today() - timedelta(days=1)
+    active_start_date = pendulum.today().subtract(days=1)
     active_task_order = TaskOrderFactory(portfolio=task_order.portfolio)
     CLINFactory(task_order=active_task_order, start_date=active_start_date)
     assert active_task_order.status == TaskOrderStatus.UNSIGNED
@@ -348,7 +348,7 @@ def test_task_orders_submit_task_order(client, user_session, task_order):
     )
     assert active_task_order.status == TaskOrderStatus.ACTIVE
 
-    upcoming_start_date = date.today() + timedelta(days=1)
+    upcoming_start_date = pendulum.today().add(days=1)
     upcoming_task_order = TaskOrderFactory(portfolio=task_order.portfolio)
     CLINFactory(task_order=upcoming_task_order, start_date=upcoming_start_date)
     assert upcoming_task_order.status == TaskOrderStatus.UNSIGNED