qt paytoedit: enable up/down keys

useful if multiline, and no harm(?) otherwise

electrum/gui/qt/completion_text_edit.py

@@ -106,10 +106,10 @@ class CompletionTextEdit(ButtonsTextEdit):
         self.completer.complete(cr)
 
     def is_special_key(self, e):
-        if self.completer != None and self.completer.popup().isVisible():
-            if e.key() in [Qt.Key_Enter, Qt.Key_Return]:
+        if self.completer and self.completer.popup().isVisible():
+            if e.key() in (Qt.Key_Enter, Qt.Key_Return):
                 return True
-        if e.key() in [Qt.Key_Tab, Qt.Key_Down, Qt.Key_Up]:
+        if e.key() == Qt.Key_Tab:
             return True
         return False