Upgrade to PySide6.

PySide2 renamed to PySide6 for Qt 6 support. It doesn't seem like
PySide2 is getting a 3.10 wheel, so upgrade to Qt 6 to prep for that.

qt_ui/windows/QLiberationWindow.py

@@ -3,12 +3,9 @@ import traceback
 import webbrowser
 from typing import Optional
 
-from PySide2.QtCore import Qt
-from PySide2.QtGui import QCloseEvent, QIcon
-from PySide2.QtWidgets import (
-    QAction,
-    QActionGroup,
-    QDesktopWidget,
+from PySide6.QtCore import Qt
+from PySide6.QtGui import QAction, QActionGroup, QCloseEvent, QGuiApplication, QIcon
+from PySide6.QtWidgets import (
     QFileDialog,
     QMainWindow,
     QMessageBox,
@@ -67,7 +64,7 @@ class QLiberationWindow(QMainWindow):
         self.initMenuBar()
         self.connectSignals()
 
-        screen = QDesktopWidget().screenGeometry()
+        screen = QGuiApplication.primaryScreen().availableSize()
         self.setGeometry(0, 0, screen.width(), screen.height())
         self.setWindowState(Qt.WindowMaximized)
 
@@ -100,7 +97,7 @@ 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))
         vbox.addWidget(hbox)