fixes access to asyncio loop via loop property of SerialTransport (#1804

)

pymodbus/transport/transport_serial.py

@@ -16,7 +16,7 @@ def __init__(self, loop, protocol, *args, **kwargs):
         """Initialize."""
         super().__init__()
         self.async_loop = loop
-        self._protocol = protocol
+        self._protocol: asyncio.BaseProtocol = protocol
         self.sync_serial = serial.serial_for_url(*args, **kwargs)
         self._closing = False
         self._write_buffer = []
@@ -37,7 +37,7 @@ def __init__(self, loop, protocol, *args, **kwargs):
     @property
     def loop(self):
         """Return asyncio event loop."""
-        return self._protocol.loop
+        return self.async_loop
 
     def get_protocol(self) -> asyncio.BaseProtocol:
         """Return protocol"""
@@ -95,7 +95,7 @@ def _read_ready(self):
         try:
             data = self.sync_serial.read(1024)
         except serial.SerialException as exc:
-            self._protocol.loop.call_soon(self._call_connection_lost, exc)
+            self.async_loop.call_soon(self._call_connection_lost, exc)
             self.close()
         else:
             if data:
@@ -130,7 +130,7 @@ def _write_ready(self):
         except (BlockingIOError, InterruptedError):
             self._write_buffer.append(data)
         except serial.SerialException as exc:
-            self._protocol.loop.call_soon(self._call_connection_lost, exc)
+            self.async_loop.call_soon(self._call_connection_lost, exc)
             self.abort()
         else:
             if nlen == len(data):