Restore support for Python 3.9

Fixes: #16

README.md

@@ -25,7 +25,8 @@ Currently supported:
 
 - Before installing, you need to have `python3` installed, with a
   working `venv` module.
-  - On Debian/Ubuntu/Raspbian, this means installing the `python3-venv` package.
+  - On Debian/Ubuntu/Raspberry Pi OS, this means installing the `python3-venv` package.
+  - Supported Python 3 versions: >= 3.9
 - Install this plugin through SignalK / npm.
 - Use the native Victron app to obtain advertisement keys for
   communicating with your Victron devices.

plugin.py

@@ -5,7 +5,7 @@
 import json
 import logging
 import sys
-from typing import Any, Callable, TypeAlias
+from typing import Any, Callable, Union
 
 from bleak.backends.device import BLEDevice
 from victron_ble.devices import (
@@ -26,16 +26,17 @@
 
 logger = logging.getLogger("signalk-victron-ble")
 
-SignalKDelta: TypeAlias = dict[str, list[dict[str, Any]]]
-SignalKDeltaValues: TypeAlias = list[dict[str, int | float | str | None]]
+# 3.9 compatible TypeAliases
+SignalKDelta = dict[str, list[dict[str, Any]]]
+SignalKDeltaValues = list[dict[str, Union[int, float, str, None]]]
 
 
 @dataclasses.dataclass
 class ConfiguredDevice:
     id: str
     mac: str
     advertisement_key: str
-    secondary_battery: str | None
+    secondary_battery: Union[str, None]
 
 
 class SignalKScanner(Scanner):