mypy: adapt new type hints to pyqt5

Ah! I'm having flashbacks to last year.

1. pyqt5 has plural enum names = define conditional type variable
2. pyqt5 doesn't wrap all the nullable things in Optional = sneakily
   make the existing overload function signature conditional.
   There might be some other was to solve this, not sure. I know we have
   qtutils.add_optional() but in this case it's complaining that the
   signature doesn't match the parent. Narrowing or widening the type of
   the returned object doesn't affect the function signature. Possibly
   we could define our own type variable MaybeOptional...

qutebrowser/completion/models/completionmodel.py

@@ -4,15 +4,22 @@
 
 """A model that proxies access to one or more completion categories."""
 
-from typing import MutableSequence, overload, Optional, Any
+from typing import MutableSequence, overload, Optional, Any, cast
 
+from qutebrowser.qt import machinery
 from qutebrowser.qt.core import Qt, QModelIndex, QAbstractItemModel, QObject
 
 from qutebrowser.utils import log, qtutils, utils
 from qutebrowser.api import cmdutils
 from qutebrowser.completion.models import BaseCategory
 
 
+if machinery.IS_QT5:
+    _FlagType = Qt.ItemFlags
+else:
+    _FlagType = Qt.ItemFlag
+
+
 class CompletionModel(QAbstractItemModel):
 
     """A model that proxies access to one or more completion categories.
@@ -75,22 +82,22 @@ def data(self, index: QModelIndex, role: int = Qt.ItemDataRole.DisplayRole) -> A
         idx = cat.index(index.row(), index.column())
         return cat.data(idx)
 
-    def flags(self, index: QModelIndex) -> Qt.ItemFlag:
+    def flags(self, index: QModelIndex) -> _FlagType:
         """Return the item flags for index.
 
         Override QAbstractItemModel::flags.
 
         Return: The item flags, or Qt.ItemFlag.NoItemFlags on error.
         """
         if not index.isValid():
-            return Qt.ItemFlag.NoItemFlags
+            return cast(_FlagType, Qt.ItemFlag.NoItemFlags)
         if index.parent().isValid():
             # item
             return (Qt.ItemFlag.ItemIsEnabled | Qt.ItemFlag.ItemIsSelectable |
                     Qt.ItemFlag.ItemNeverHasChildren)
         else:
             # category
-            return Qt.ItemFlag.NoItemFlags
+            return cast(_FlagType, Qt.ItemFlag.NoItemFlags)
 
     def index(self, row: int, col: int, parent: QModelIndex = QModelIndex()) -> QModelIndex:
         """Get an index into the model.
@@ -113,9 +120,15 @@ def index(self, row: int, col: int, parent: QModelIndex = QModelIndex()) -> QMod
     def parent(self, index: QModelIndex) -> QModelIndex:
         ...
 
-    @overload
-    def parent(self) -> Optional[QObject]:
-        ...
+    if machinery.IS_QT5:
+        @overload
+        def parent(self) -> QObject:
+            ...
+
+    else:
+        @overload
+        def parent(self) -> Optional[QObject]:
+            ...
 
     def parent(self, index=None):
         """Get an index to the parent of the given index.