Fix UnboundLocalError in MIDI handler when pyserial is missing

- Remove serial.SerialException from except clause since 'serial'
  is not defined when import serial fails, causing UnboundLocalError
- Catch only ImportError and OSError

src/midi/handler.py

@@ -165,7 +165,7 @@ class UARTMIDI(MIDIInterface):
             )
             logger.info("UART MIDI opened on %s", self._port_name)
             return True
-        except (ImportError, OSError, serial.SerialException) as e:
+        except (ImportError, OSError) as e:
             logger.error("UART MIDI open failed: %s", e)
             return False