qt: allow QR codes to store a bit more data

by decreasing error correction (about ~26% larger max payload)

electrum/gui/qt/qrcodewidget.py

@@ -27,7 +27,11 @@ class QRCodeWidget(QWidget):
         if self.data != data:
             self.data = data
         if self.data:
-            self.qr = qrcode.QRCode()
+            self.qr = qrcode.QRCode(
+                error_correction=qrcode.constants.ERROR_CORRECT_L,
+                box_size=10,
+                border=0,
+            )
             self.qr.add_data(self.data)
             if not self.fixedSize:
                 k = len(self.qr.get_matrix())

electrum/gui/qt/transaction_dialog.py

@@ -28,6 +28,7 @@ import copy
 import datetime
 import json
 import traceback
+from typing import TYPE_CHECKING
 
 from PyQt5.QtCore import QSize, Qt
 from PyQt5.QtGui import QTextCharFormat, QBrush, QFont
@@ -47,6 +48,9 @@ from electrum.logging import get_logger
 from .util import (MessageBoxMixin, read_QIcon, Buttons, CopyButton,
                    MONOSPACE_FONT, ColorScheme, ButtonsLineEdit)
 
+if TYPE_CHECKING:
+    from .main_window import ElectrumWindow
+
 
 SAVE_BUTTON_ENABLED_TOOLTIP = _("Save transaction offline")
 SAVE_BUTTON_DISABLED_TOOLTIP = _("Please sign this transaction in order to save it")
@@ -83,7 +87,7 @@ class TxDialog(QDialog, MessageBoxMixin):
             self.tx.deserialize()
         except BaseException as e:
             raise SerializationError(e)
-        self.main_window = parent
+        self.main_window = parent  # type: ElectrumWindow
         self.wallet = parent.wallet
         self.prompt_if_unsaved = prompt_if_unsaved
         self.saved = False