Add `pyqtgraph` stub

[TiborVoelcker]

The pyqtgraph library has some dynamically inherited methods for some classes, which make my linter and type checker mad.

For example, the GraphicsLayoutWidget is a PyQt widget, which has a GraphicsLayout as its central item. For convenience, it then adds a bunch of methods from the GraphicsLayout class to itself, which are then missed by the type checker.

That is why I would like to request to add the stub files for this third-party library.