Turn warnings into errors; acknowledge existing errors

We have two warnings on the repo at the moment, both from dependencies
(so outside of our control). One is from botocore
(boto/boto3#3889) and the other is from
sqlalchemy. Both are Python 3.12 DeprecationWarnings about
datetime.datetime.utcnow and datetime.datetime.utcfromtimestamp being
scheduled for removal in future python versions.

pytest.ini

@@ -2,3 +2,7 @@
 env =
     FLASK_ENV=unit_test
     FLASK_DEBUG=1
+filterwarnings =
+    error
+    ignore:datetime.datetime.utcfromtimestamp:DeprecationWarning:sqlalchemy
+    ignore:datetime.datetime.utcnow:DeprecationWarning:botocore