Fix TypeError on Python <3.10: add from __future__ import annotations

dict[str, Any] and X | Y union syntax require Python 3.10+. Containers
running centos7/ubuntu20/rocky9 base images ship Python 3.6-3.9 and
crashed on import. Adding the __future__ import makes all annotations
lazy strings, compatible back to Python 3.7.

Affected: templates/decnet_logging.py (+ all 22 template copies),
decnet/logging/syslog_formatter.py, decnet/logging/file_handler.py

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

decnet/logging/file_handler.py

@@ -1,3 +1,4 @@
+from __future__ import annotations
 """
 Rotating file handler for DECNET syslog output.