convert PySide2 to PySide6 in tests

tests/controller/test_processing.py

@@ -12,7 +12,7 @@ from sohstationviewer.controller.processing import (
     get_data_type_from_file
 )
 from sohstationviewer.database.extract_data import get_signature_channels
-from PySide2 import QtWidgets
+from PySide6 import QtWidgets
 
 
 TEST_DATA_DIR = Path(__file__).resolve().parent.parent.joinpath('test_data')