set AA_EnableHighDpiScaling attribute early

This avoids Qt 5.12.4 and newer spewing a warning:

> Attribute Qt::AA_EnableHighDpiScaling must be set before QCoreApplication is created.

src/sakia/app.py

@@ -114,7 +114,6 @@ class Application(QObject):
 
     @classmethod
     def startup(cls, argv, qapp, loop):
-        qapp.setAttribute(Qt.AA_EnableHighDpiScaling, True)
         options = SakiaOptions.from_arguments(argv)
         app_data = AppDataFile.in_config_path(options.config_path).load_or_init()
         app = cls(

src/sakia/main.py

@@ -157,6 +157,12 @@ def cancel_connection(button=None):
 def main():
     #  activate ctrl-c interrupt
     signal.signal(signal.SIGINT, signal.SIG_DFL)
+
+    try:
+        QApplication.setAttribute(Qt.AA_EnableHighDpiScaling)
+    except AttributeError:  # Attribute only exists for Qt>=5.6.
+        pass
+
     sakia = QApplication(sys.argv)
 
     sys.excepthook = exception_handler