switch from PyQt6 to PySide6

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

highlight_regex.py

@@ -3,7 +3,7 @@ from typing import Optional
 from highlight import Highlight
 from highlighted_range import HighlightedRange
 from line import Line
-from PyQt6.QtGui import *
+from PySide6.QtGui import QBrush, QColor
 
 from typing import List
 import re