fix missing import

electrum/plugins/payserver/qt.py

@@ -24,7 +24,9 @@
 # SOFTWARE.
 
 from functools import partial
+from typing import TYPE_CHECKING
 from PyQt5 import QtWidgets
+
 from electrum.i18n import _
 from electrum.plugin import hook
 from electrum.gui.qt.util import WindowModalDialog, OkButton, Buttons, EnterButton, webopen