Small fixes for marks.

Mark two callbacks with pyqtSlot as appropriate.
Return False instead of raising NotImplementedException to avoid pylint
identifying MarkKeyParser as abstract.

qutebrowser/keyinput/modeparsers.py

@@ -279,10 +279,11 @@ class MarkKeyParser(keyparser.BaseKeyParser):
 
         return True
 
+    @pyqtSlot(str)
     def on_keyconfig_changed(self, mode):
         """MarkKeyParser has no config section (no bindable keys)."""
         pass
 
     def execute(self, cmdstr, _keytype, count=None):
         """Should never be called on MarkKeyParser."""
-        raise NotImplementedError
+        assert False

qutebrowser/mainwindow/tabbedbrowser.py

@@ -673,6 +673,7 @@ class TabbedBrowser(tabwidget.TabWidget):
         if key.isupper() and key in self._global_marks:
             point, url = self._global_marks[key]
 
+            @pyqtSlot(bool)
             def callback(ok):
                 if ok:
                     self.cur_load_finished.disconnect(callback)