Use __future__ to make old python happy

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

tests/test_proxy_environment.py

@@ -3,6 +3,8 @@
 
 """Test ngclient ProxyEnvironment"""
 
+from __future__ import annotations
+
 import sys
 import unittest
 from unittest.mock import Mock, patch

tuf/ngclient/_internal/proxy.py

@@ -3,6 +3,8 @@
 
 """Proxy environment variable handling with Urllib3"""
 
+from __future__ import annotations
+
 from typing import Any
 from urllib.request import getproxies