Update to PySide6.

It sounds like PySide2 will not be moving to Python 3.11, so we're stuck
on 3.10 without this. Upgrading to a newer Qt also fixes some high DPI
bugs (the file browser dialog for save/load is no longer tiny on 4k).

https://github.com/pyinstaller/pyinstaller/issues/5414 previously
blocked this, but the bug appears to be fixed now.

qt_ui/windows/QLiberationWindow.py

@@ -3,13 +3,10 @@ import traceback
 import webbrowser
 from typing import Optional
 
-from PySide2.QtCore import QSettings, Qt, Signal
-from PySide2.QtGui import QCloseEvent, QIcon
-from PySide2.QtWidgets import (
-    QAction,
-    QActionGroup,
+from PySide6.QtCore import QSettings, Qt, Signal
+from PySide6.QtGui import QAction, QActionGroup, QCloseEvent, QGuiApplication, QIcon
+from PySide6.QtWidgets import (
     QApplication,
-    QDesktopWidget,
     QFileDialog,
     QMainWindow,
     QMessageBox,
@@ -97,7 +94,7 @@ class QLiberationWindow(QMainWindow):
 
         # Default to maximized on the main display if we don't have any persistent
         # configuration.
-        screen = QDesktopWidget().screenGeometry()
+        screen = QGuiApplication.primaryScreen().availableSize()
         self.setGeometry(0, 0, screen.width(), screen.height())
         self.setWindowState(Qt.WindowMaximized)
 
@@ -133,7 +130,7 @@ class QLiberationWindow(QMainWindow):
         vbox.setSizes([600, 100])
 
         vbox = QVBoxLayout()
-        vbox.setMargin(0)
+        vbox.setContentsMargins(0, 0, 0, 0)
         vbox.addWidget(QTopPanel(self.game_model, self.sim_controller, ui_flags))
         vbox.addWidget(hbox)