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_user.py

@@ -1,6 +1,6 @@
 import pytest
 from sqlalchemy.exc import InternalError
-from datetime import datetime
+import pendulum
 
 from atst.database import db
 from atst.domain.users import Users
@@ -44,7 +44,7 @@ def test_deleted_application_roles_are_ignored(session):
 
 def test_does_not_log_user_update_when_updating_last_login(mock_logger):
     user = UserFactory.create()
-    user.last_login = datetime.now()
+    user.last_login = pendulum.now(tz="utc")
     db.session.add(user)
     db.session.commit()
     assert "Audit Event update" not in mock_logger.messages