Fix python strict type checking in Python 3.9+

Python 3.9+ started doing strict enforcement of types.  This results in the GUI function methods not functioning properly in modern Python environments.

Fixes torproject#628

Thanks to @icceland in bug 628 for the fix stated in the bug.

torbrowser_launcher/__init__.py

@@ -90,8 +90,8 @@ def main():
     desktop = app.desktop()
     window_size = gui.size()
     gui.move(
-        (desktop.width() - window_size.width()) / 2,
-        (desktop.height() - window_size.height()) / 2,
+        (desktop.width() - window_size.width()) // 2,
+        (desktop.height() - window_size.height()) // 2,
     )
     gui.show()