[dbmanager] Fix exception on older Qt builds

Fixes #19309

(cherry-picked from 9d8202b)

Author: nyalldawson

python/plugins/db_manager/dlg_sql_window.py

@@ -96,7 +96,11 @@ def __init__(self, iface, db, parent=None):
         self.btnCancel.setText(self.tr("Cancel (ESC)"))
         self.btnCancel.setEnabled(False)
         self.btnCancel.clicked.connect(self.executeSqlCanceled)
-        self.btnCancel.setShortcut(QKeySequence.Cancel)
+        try:
+            self.btnCancel.setShortcut(QKeySequence.Cancel)
+        except AttributeError:
+            # QKeySequence.Cancel only available in Qt >= 5.6
+            pass
         self.progressBar.setEnabled(False)
         self.progressBar.setRange(0, 100)
         self.progressBar.setValue(0)