mypy: Fix new issues in models.completionmodel

qutebrowser/completion/models/completionmodel.py

@@ -43,7 +43,8 @@ class CompletionModel(QAbstractItemModel):
     def __init__(self, *, column_widths=(30, 70, 0), parent=None):
         super().__init__(parent)
         self.column_widths = column_widths
-        self._categories = []  # type: typing.Sequence[QAbstractItemModel]
+        self._categories = [
+        ]  # type: typing.MutableSequence[QAbstractItemModel]
 
     def _cat_from_idx(self, index):
         """Return the category pointed to by the given index.
@@ -182,10 +183,11 @@ class CompletionModel(QAbstractItemModel):
         # WORKAROUND:
         # layoutChanged is broken in PyQt 5.7.1, so we must use metaObject
         # https://www.riverbankcomputing.com/pipermail/pyqt/2017-January/038483.html
-        self.metaObject().invokeMethod(self, "layoutAboutToBeChanged")
+        self.metaObject().invokeMethod(self,  # type: ignore
+                                       "layoutAboutToBeChanged")
         for cat in self._categories:
             cat.set_pattern(pattern)
-        self.metaObject().invokeMethod(self, "layoutChanged")
+        self.metaObject().invokeMethod(self, "layoutChanged")  # type: ignore
 
     def first_item(self):
         """Return the index of the first child (non-category) in the model."""