mypy: cast variable only on Qt5

We only have to cast these variables on Qt5. If we cast them on Qt6 mypy
warns of a redundant cast. If we ignore that warning mypy complains of a
redundant ignore on Qt5.

Add a conditional on `machinery.IS_QT5` and only cast it required.

But also move that conditional out to a utility function to avoid
duplicating code in the function doing the actual logic.

I'm re-using the `_T` generic TypeVar defined above. Not sure if it's
proper to re-use them or not. Doesn't python have a better way of
defining generics now?

I added the `valid-type` ignore because mypy was complaining:

    Variable "to_type" is not valid as a type

I'm not sure if there is some way to do it better, but `reveal_type()`
says that the call site is getting the right type back in the end.

qutebrowser/completion/models/completionmodel.py

@@ -4,7 +4,7 @@
 
 """A model that proxies access to one or more completion categories."""
 
-from typing import overload, Optional, Any, cast
+from typing import overload, Optional, Any
 from collections.abc import MutableSequence
 
 from qutebrowser.qt import machinery
@@ -91,14 +91,14 @@ class CompletionModel(QAbstractItemModel):
         Return: The item flags, or Qt.ItemFlag.NoItemFlags on error.
         """
         if not index.isValid():
-            return cast(_FlagType, Qt.ItemFlag.NoItemFlags)
+            return qtutils.maybe_cast(_FlagType, machinery.IS_QT5, Qt.ItemFlag.NoItemFlags)
         if index.parent().isValid():
             # item
             return (Qt.ItemFlag.ItemIsEnabled | Qt.ItemFlag.ItemIsSelectable |
                     Qt.ItemFlag.ItemNeverHasChildren)
         else:
             # category
-            return cast(_FlagType, Qt.ItemFlag.NoItemFlags)
+            return qtutils.maybe_cast(_FlagType, machinery.IS_QT5, Qt.ItemFlag.NoItemFlags)
 
     def index(self, row: int, col: int, parent: QModelIndex = QModelIndex()) -> QModelIndex:
         """Get an index into the model.

qutebrowser/misc/nativeeventfilter.py

@@ -7,7 +7,7 @@
 This entire file is a giant WORKAROUND for https://bugreports.qt.io/browse/QTBUG-114334.
 """
 
-from typing import Union, cast, Optional
+from typing import Union, Optional
 import enum
 import ctypes
 import ctypes.util
@@ -16,7 +16,7 @@ from qutebrowser.qt import sip, machinery
 from qutebrowser.qt.core import QAbstractNativeEventFilter, QByteArray, qVersion
 
 from qutebrowser.misc import objects
-from qutebrowser.utils import log
+from qutebrowser.utils import log, qtutils
 
 
 # Needs to be saved to avoid garbage collection
@@ -104,8 +104,8 @@ class NativeEventFilter(QAbstractNativeEventFilter):
     #
     # Tuple because PyQt uses the second value as the *result out-pointer, which
     # according to the Qt documentation is only used on Windows.
-    _PASS_EVENT_RET = (False, cast(_PointerRetType, 0))
-    _FILTER_EVENT_RET = (True, cast(_PointerRetType, 0))
+    _PASS_EVENT_RET = (False, qtutils.maybe_cast(_PointerRetType, machinery.IS_QT6, 0))
+    _FILTER_EVENT_RET = (True, qtutils.maybe_cast(_PointerRetType, machinery.IS_QT6, 0))
 
     def __init__(self) -> None:
         super().__init__()

qutebrowser/utils/qtutils.py

@@ -749,3 +749,14 @@ else:
         return obj
 
     QT_NONE: None = None
+
+
+def maybe_cast(to_type: type[_T], do_cast: bool, value: Any) -> _T:
+    """Cast `value` to `to_type` only if `do_cast` is true."""
+    if do_cast:
+        return cast(
+            to_type,  # type: ignore[valid-type]
+            value,
+        )
+
+    return value