Allow advanced annotations in Python 3.8

Author: jwindgassen

jupyter_server_proxy/standalone/__init__.py

@@ -1,3 +1,5 @@
+from __future__ import annotations  # For Python 3.8 compatibility
+
 import argparse
 import logging
 import os

jupyter_server_proxy/standalone/proxy.py

@@ -1,3 +1,5 @@
+from __future__ import annotations  # For Python 3.8 compatibility
+
 import os
 import re
 import ssl