migrate from PyQt6 to PySide6, remove unneeded dependencies

src/ui/widgets/MessageCalendar.py

@@ -3,9 +3,9 @@ from typing import Any
 
 import darkdetect
 import loguru
-from PyQt6 import QtCore, QtWidgets
-from PyQt6.QtCore import QDate
-from PyQt6.QtGui import QColor, QPen
+from PySide6 import QtCore, QtWidgets
+from PySide6.QtCore import QDate
+from PySide6.QtGui import QColor, QPen
 
 from src import LOG_DIR
 from src.backend import Database