[electrum] fix the transaction dialog for Qt6

Summary:
Fixes the following error with pyqt6:

File "/home/pierre/dev/bitcoin-abc/electrum/electrumabc_gui/qt/transaction_dialog.py", line 225, in __init__
    b.setShortcut(QKeySequence(Qt.ALT + Qt.Key_Q))
                               ~~~~~~~^~~~~~~~~~
TypeError: unsupported operand type(s) for +: 'Modifier' and 'Key'

Test Plan:
With pyqt5 and pyqt6 (QT_API=pyqt6 ./electrum-abc):
In the tx history, right click on a tx and select Details in the menu. Check for no error and that the Alt+Q shortcut works

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18345