switch from PyQt6 to PySide6

PySide6 uses LGPL instead of GPL, which is much nicer to work with.

highlight_selection.py

@@ -3,8 +3,8 @@ from typing import Optional, List
 from highlight import Highlight
 from highlighted_range import HighlightedRange
 from line import Line
-from PyQt6.QtCore import *
-from PyQt6.QtGui import *
+from PySide6.QtCore import Qt
+from PySide6.QtGui import QBrush, QColor
 
 from settings import Settings