Make PoP start and end dates inclusive.

Also removes the clock class.

Makes PoP date ranges inclusive such that a task order with:
-  a start date on or after the current date
and
- an end date on or before the current date
should be considered valid.

This commit also removes the Clock class. This class had two methods as
shortcuts for common uses of pendlum functions. But it wasn't being used
in very many places, and it took up about the same space as

    from pendulum import today()
    ...
    today(tz="UTC").date()

If we want to add this back in, it might be a good idea to extend it for
other time functions we have sprinkled around, like the random date
functions in our tests

atst/models/task_order.py

@@ -9,7 +9,7 @@ from atst.models.base import Base
 import atst.models.types as types
 import atst.models.mixins as mixins
 from atst.models.attachment import Attachment
-from atst.utils.clock import Clock
+from pendulum import today
 
 
 class Status(Enum):
@@ -117,17 +117,17 @@ class TaskOrder(Base, mixins.TimestampsMixin):
 
     @property
     def status(self):
-        today = Clock.today()
+        todays_date = today(tz="UTC").date()
 
         if not self.is_completed and not self.is_signed:
             return Status.DRAFT
         elif self.is_completed and not self.is_signed:
             return Status.UNSIGNED
-        elif today < self.start_date:
+        elif todays_date < self.start_date:
             return Status.UPCOMING
-        elif today >= self.end_date:
+        elif todays_date > self.end_date:
             return Status.EXPIRED
-        elif self.start_date <= today < self.end_date:
+        elif self.start_date <= todays_date <= self.end_date:
             return Status.ACTIVE
 
     @property
@@ -141,7 +141,7 @@ class TaskOrder(Base, mixins.TimestampsMixin):
     @property
     def days_to_expiration(self):
         if self.end_date:
-            return (self.end_date - Clock.today()).days
+            return (self.end_date - today(tz="UTC").date()).days
 
     @property
     def total_obligated_funds(self):