Suggestion: use qtpy for abstracting the qt implementation

[mbuijs]

From the Spyder project there is the qtpy abstraction layer that lets you write code as if PyQt5 is imported, but run with Qt4 under the hood. This can simplify some parts of the code, taking away some if statements. Additionally the ambiguous import warnings that Spyder gives when opening the project ("QListWidget exists in both PyQt4 and PyQt5") would be gone also.

[wkentaro]

How can it handle .qrc file?
https://github.com/wkentaro/labelme/blob/master/labelme/resources.qrc

[wkentaro]

54863e2 made the .qrc file no longer necessary.

[wkentaro]

Do you have some idea what library is better for this?
There is a comparison matrix at mottosso/Qt.py:
https://github.com/mottosso/Qt.py#projects-similar-to-qtpy

[wkentaro]

@jbvo Thank you for the suggestion! This is supported via #130 .