indicate that REST API timestamps are UTC

use iso8601 Z suffix for UTC timestamps

use dateutil to parse dates from proxy, as well

even though CHP uses iso8601 UTC timestamps, we no longer assume CHP, so use more general parsing

in our db we are stuck with naïve datetime objects, so use those internally.
But ensure we put 'Z' on timestamps we ship externally

jupyterhub/utils.py

@@ -6,6 +6,7 @@
 import asyncio
 from binascii import b2a_hex
 import concurrent.futures
+from datetime import datetime, timezone
 import random
 import errno
 import hashlib
@@ -15,7 +16,6 @@ import os
 import socket
 import sys
 import threading
-from threading import Thread
 import uuid
 import warnings
 
@@ -39,6 +39,16 @@ ISO8601_ms = '%Y-%m-%dT%H:%M:%S.%fZ'
 ISO8601_s = '%Y-%m-%dT%H:%M:%SZ'
 
 
+def isoformat(dt):
+    """Render a datetime object as an ISO 8601 UTC timestamp
+
+    Naïve datetime objects are assumed to be UTC
+    """
+    if dt.tzinfo:
+        dt = dt.astimezone(timezone.utc).replace(tzinfo=None)
+    return dt.isoformat() + 'Z'
+
+
 def can_connect(ip, port):
     """Check if we can connect to an ip:port.