Make QWebEngineView.setPage() parameter optional (#212)

This change allows the page passed to setPage() to be None, which is a
valid technique and often needed.

PyQt5-stubs/QtWebEngineWidgets.pyi

@@ -1037,5 +1037,5 @@ class QWebEngineView(QtWidgets.QWidget):
     def load(self, url: QtCore.QUrl) -> None: ...
     @typing.overload
     def load(self, request: QtWebEngineCore.QWebEngineHttpRequest) -> None: ...
-    def setPage(self, page: QWebEnginePage) -> None: ...
+    def setPage(self, page: typing.Optional[QWebEnginePage]) -> None: ...
     def page(self) -> QWebEnginePage: ...