Change tango to support 3.12: AttributeProxy::_event_processor to be coroutine

[evalott100]

Using python 3.12, we get the following deprecation warning:

--------------------------------------------------------- Captured stdout setup ---------------------------------------------------------
Ready to accept request
--------------------------------------------------------- Captured stderr setup ---------------------------------------------------------
Can't create notifd event supplier. Notifd event not available
--------------------------------------------------------- Captured stderr call ----------------------------------------------------------
push_event generated the following python exception:
Traceback (most recent call last):
  File "/scratch/twj43146/Programming/ophyd-async/.venv/lib/python3.12/site-packages/tango/green.py", line 249, in greener
    return executor.submit(fn, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/scratch/twj43146/Programming/ophyd-async/.venv/lib/python3.12/site-packages/tango/asyncio_executor.py", line 214, in submit
    warnings.warn(
DeprecationWarning: Sync AttributeProxy._event_processor function called: support of sync functions in PyTango's Asyncio mode is deprecated. Use 'async def' instead of 'def'.

This was causing test failures. We ignored them in #655, but we should alter this method to be a coroutine and we should remove the test skipping. The tests may also be faulty on 3.12 for other reasons.

Callback methods on AttributeProxy cannot easily be made into coroutines since SignalBackend does not currently support async callbacks.