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/portfolios/test_invitations.py

@@ -1,4 +1,4 @@
-import datetime
+import pendulum
 from unittest.mock import Mock
 
 from flask import url_for
@@ -123,7 +123,7 @@ def test_user_accepts_expired_invite(client, user_session):
         user_id=user.id,
         role=ws_role,
         status=InvitationStatus.REJECTED_EXPIRED,
-        expiration_time=datetime.datetime.now() - datetime.timedelta(seconds=1),
+        expiration_time=pendulum.now(tz="utc").subtract(seconds=1),
     )
     user_session(user)
     response = client.get(
@@ -143,7 +143,7 @@ def test_revoke_invitation(client, user_session):
         user_id=user.id,
         role=ws_role,
         status=InvitationStatus.REJECTED_EXPIRED,
-        expiration_time=datetime.datetime.now() - datetime.timedelta(seconds=1),
+        expiration_time=pendulum.now(tz="utc").subtract(seconds=1),
     )
     user_session(portfolio.owner)
     response = client.post(
@@ -169,7 +169,7 @@ def test_user_can_only_revoke_invites_in_their_portfolio(client, user_session):
         user_id=user.id,
         role=portfolio_role,
         status=InvitationStatus.REJECTED_EXPIRED,
-        expiration_time=datetime.datetime.now() - datetime.timedelta(seconds=1),
+        expiration_time=pendulum.now(tz="utc").subtract(seconds=1),
     )
     user_session(portfolio.owner)
     response = client.post(
@@ -199,7 +199,7 @@ def test_user_can_only_resend_invites_in_their_portfolio(
         user_id=user.id,
         role=portfolio_role,
         status=InvitationStatus.REJECTED_EXPIRED,
-        expiration_time=datetime.datetime.now() - datetime.timedelta(seconds=1),
+        expiration_time=pendulum.now(tz="utc").subtract(seconds=1),
     )
     user_session(portfolio.owner)
     response = client.post(