bluez: Allow accessing Bleak from multiple individual event loops

Previous event loop needs to be closed for this to work, as the rest
of the DBus access code doesn't support concurrent access from more than one
MessageBus instance.

However this allows creating per-test Bleak classes with pytest-asyncio that
don't overlap each other, and possibly other use cases as well.

bleak/backends/bluezdbus/manager.py

@@ -20,6 +20,7 @@
     Optional,
     Set,
     cast,
+    Union,
 )
 
 from dbus_fast import BusType, Message, MessageType, Variant
@@ -846,12 +847,30 @@ def _run_advertisement_callbacks(
 async def get_global_bluez_manager() -> BlueZManager:
     """
     Gets the initialized global BlueZ manager instance.
+
+    If a BlueZ manager was previously initialized and associated with an event loop which is now closed,
+    then a new instance will be created to replace it.
     """
 
-    if not hasattr(get_global_bluez_manager, "instance"):
-        setattr(get_global_bluez_manager, "instance", BlueZManager())
+    loop = asyncio.get_event_loop()
 
-    instance: BlueZManager = getattr(get_global_bluez_manager, "instance")
+    if not hasattr(get_global_bluez_manager, "instance"):
+        get_global_bluez_manager.instance = BlueZManager()
+        get_global_bluez_manager.loop = loop
+
+    instance: BlueZManager = get_global_bluez_manager.instance
+    stored_loop: Union(
+        asyncio.SelectorEventLoop, asyncio.ProactorEventLoop, None
+    ) = get_global_bluez_manager.loop
+
+    if loop != stored_loop:
+        if stored_loop.is_closed():
+            logger.debug("Replacing BlueZManager for new event loop")
+            instance = BlueZManager()
+            get_global_bluez_manager.instance = instance
+            get_global_bluez_manager.loop = loop
+        else:
+            logger.warning("Accessing BlueZ from two open event loops is not supported")
 
     await instance.async_init()