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/forms/test_task_order.py

@@ -1,4 +1,4 @@
-import datetime
+import pendulum
 from dateutil.relativedelta import relativedelta
 from flask import current_app as app
 
@@ -17,8 +17,8 @@ def test_clin_form_jedi_clin_type():
 
 
 def test_clin_form_start_date_before_end_date():
-    invalid_start = datetime.date(2020, 12, 12)
-    invalid_end = datetime.date(2020, 1, 1)
+    invalid_start = pendulum.date(2020, 12, 12)
+    invalid_end = pendulum.date(2020, 1, 1)
     invalid_clin = factories.CLINFactory.create(
         start_date=invalid_start, end_date=invalid_end
     )
@@ -28,8 +28,8 @@ def test_clin_form_start_date_before_end_date():
         translate("forms.task_order.pop_errors.date_order")
         in clin_form.start_date.errors
     )
-    valid_start = datetime.date(2020, 1, 1)
-    valid_end = datetime.date(2020, 12, 12)
+    valid_start = pendulum.date(2020, 1, 1)
+    valid_end = pendulum.date(2020, 12, 12)
     valid_clin = factories.CLINFactory.create(
         start_date=valid_start, end_date=valid_end
     )
@@ -81,8 +81,8 @@ def test_clin_form_obligated_greater_than_total():
     invalid_clin = factories.CLINFactory.create(
         total_amount=0,
         obligated_amount=1,
-        start_date=datetime.date(2019, 9, 15),
-        end_date=datetime.date(2020, 9, 14),
+        start_date=pendulum.date(2019, 9, 15),
+        end_date=pendulum.date(2020, 9, 14),
     )
     invalid_clin_form = CLINForm(obj=invalid_clin)
     assert not invalid_clin_form.validate()
@@ -95,8 +95,8 @@ def test_clin_form_dollar_amounts_out_of_range():
     invalid_clin = factories.CLINFactory.create(
         total_amount=-1,
         obligated_amount=1000000001,
-        start_date=datetime.date(2019, 9, 15),
-        end_date=datetime.date(2020, 9, 14),
+        start_date=pendulum.date(2019, 9, 15),
+        end_date=pendulum.date(2020, 9, 14),
     )
     invalid_clin_form = CLINForm(obj=invalid_clin)
     assert not invalid_clin_form.validate()