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.

atst/models/clin.py

@@ -9,7 +9,7 @@ from sqlalchemy import (
     String,
 )
 from sqlalchemy.orm import relationship
-from datetime import date
+import pendulum
 
 from atst.models.base import Base
 import atst.models.mixins as mixins
@@ -75,5 +75,5 @@ class CLIN(Base, mixins.TimestampsMixin):
     @property
     def is_active(self):
         return (
-            self.start_date <= date.today() <= self.end_date
+            self.start_date <= pendulum.today() <= self.end_date
         ) and self.task_order.signed_at

atst/models/mixins/invites.py

@@ -1,4 +1,4 @@
-import datetime
+import pendulum
 from enum import Enum
 import secrets
 
@@ -90,7 +90,7 @@ class InvitesMixin(object):
     @property
     def is_expired(self):
         return (
-            datetime.datetime.now(self.expiration_time.tzinfo) > self.expiration_time
+            pendulum.now(tz=self.expiration_time.tzinfo) > self.expiration_time
             and not self.status == Status.ACCEPTED
         )