Experiment with descriptors used by Django

[JukkaL]

Try to write some minimal stubs that simulate how Django uses descriptors, and see if we can type check code using them. Depending on the results, we may want to add some test cases that reflect these use cases, or improve the descriptor implementation.

Follow-up to #2266.

[ilevkivskyi]

There are now decent stubs for Django (that use descriptors extensively), and there were no bug reports so far, so I think this can be closed.