fix: remove datetime.UTC from mqtt_handler and add 3.10 compat test

Replace the try/except UTC shim in mqtt_handler.py with timezone.utc
directly, consistent with the api_endpoints.py fix. Add a static AST
scan that fails if datetime.UTC is reintroduced anywhere in the codebase,
guarding against future regressions on Python 3.10 (LuckFox Pico Ultra).

Co-Authored-By: Zindello <josh@zindello.com.au>

repeater/data_acquisition/mqtt_handler.py

@@ -4,19 +4,12 @@ import json
 import logging
 import string
 import threading
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from typing import Any, Callable, Dict, List, Optional
 
 import paho.mqtt.client as mqtt
 from nacl.signing import SigningKey
 
-# Try to import datetime.UTC (Python 3.11+) otherwise fallback to timezone.utc
-try:
-    from datetime import UTC
-except Exception:
-    from datetime import timezone
-    UTC = timezone.utc
-
 from repeater import __version__, config
 from repeater.presets import get_preset
 
@@ -268,7 +261,7 @@ class _BrokerConnection:
 
     def _generate_jwt(self) -> str:
         """Generate MeshCore-style Ed25519 JWT token"""
-        now = datetime.now(UTC)
+        now = datetime.now(timezone.utc)
 
         header = {"alg": "Ed25519", "typ": "JWT"}
 
@@ -448,7 +441,7 @@ class _BrokerConnection:
             else:
                 logger.info(f"No credentials set for {self.broker['name']} (JWT auth disabled and no username/password provided)")
             
-            self._connect_time = datetime.now(UTC)
+            self._connect_time = datetime.now(timezone.utc)
             
         except Exception as e:
             logger.error(f"Failed to set JWT credentials for {self.broker['name']}: {e}")
@@ -550,7 +543,7 @@ class _BrokerConnection:
         """Check if connection should be reconnected due to JWT expiry (at 80% of lifetime)"""
         if not self._connect_time:
             return False
-        elapsed = (datetime.now(UTC) - self._connect_time).total_seconds()
+        elapsed = (datetime.now(timezone.utc) - self._connect_time).total_seconds()
         expiry_seconds = self.jwt_expiry_minutes * 60
         # Stagger refresh by 5% per broker to prevent simultaneous disconnects
         # Broker 0: 80%, Broker 1: 85%, Broker 2: 90%, etc.
@@ -906,7 +899,7 @@ class MeshCoreToMqttPusher:
     # Packet helpers
     # ----------------------------------------------------------------
     def _process_packet(self, pkt: dict) -> dict:
-        return {"timestamp": datetime.now(UTC).isoformat(), "origin_id": self.public_key, **pkt}
+        return {"timestamp": datetime.now(timezone.utc).isoformat(), "origin_id": self.public_key, **pkt}
 
     def publish_packet(self, pkt: dict, subtopic="packets", retain=False):
         return self.publish(subtopic, self._process_packet(pkt), retain)
@@ -941,7 +934,7 @@ class MeshCoreToMqttPusher:
 
         status = {
             "status": state,
-            "timestamp": datetime.now(UTC).isoformat(),
+            "timestamp": datetime.now(timezone.utc).isoformat(),
             "origin": origin or self.node_name,
             "origin_id": self.public_key,
             "model": "PyMC-Repeater",

tests/test_python310_compat.py

@@ -0,0 +1,43 @@
+"""Regression tests guarding against Python 3.10 compatibility breakage.
+
+pyMC Repeater supports Python 3.10+ (LuckFox Pico Ultra ships with 3.10).
+These tests scan the source tree statically so regressions are caught in CI
+without needing a 3.10 interpreter in the test environment.
+"""
+
+import ast
+import sys
+from pathlib import Path
+
+_REPEATER_ROOT = Path(__file__).parent.parent / "repeater"
+
+
+def _py_files():
+    return [p for p in _REPEATER_ROOT.rglob("*.py") if ".pyc" not in str(p)]
+
+
+def test_minimum_python_version():
+    """Fail fast if the test environment itself is below the minimum supported version."""
+    assert sys.version_info >= (3, 10), (
+        f"Python 3.10+ required, running {sys.version_info.major}.{sys.version_info.minor}"
+    )
+
+
+def test_no_datetime_utc():
+    """`datetime.UTC` was added in 3.11 — `timezone.utc` must be used instead."""
+    violations = []
+    for path in _py_files():
+        try:
+            tree = ast.parse(path.read_text(encoding="utf-8"))
+        except SyntaxError:
+            continue
+        for node in ast.walk(tree):
+            if isinstance(node, ast.ImportFrom) and node.module == "datetime":
+                if any(alias.name == "UTC" for alias in node.names):
+                    rel = path.relative_to(_REPEATER_ROOT.parent)
+                    violations.append(f"  {rel}:{node.lineno}")
+
+    assert not violations, (
+        "datetime.UTC (Python 3.11+) found in the following files — "
+        "use timezone.utc instead:\n" + "\n".join(violations)
+    )