Testing stubs against Django source code

[mkurnikov]

We need to start using actual Django sources to test stubs. For that, we have to merge our .pyi files inside the .py files of Django.

There's a library called LibCST https://github.com/Instagram/LibCST, which allows traversing and modifying Python source code. It's new, Python 3 only, covered with type annotations and well-maintained by Instagram. I think we should use it as a base for our merging tool, instead of
https://github.com/ambv/retype
https://github.com/google/pytype/tree/master/pytype/tools/merge_pyi
which are both lib2to3 based.

I used it in the #288 to make an example script to add stubs versioning.

It even has some built-in support for this particular task via https://libcst.readthedocs.io/en/latest/codemods.html#libcst.codemod.visitors.ApplyTypeAnnotationsVisitor

The first step here is to make a simple script that uses this visitor to apply stubs. Later, the ApplyTypeAnnotationsVisitor will be subclassed with some custom code, if we'd need any special casing.

@kszmigiel Could you take a look at it? Maybe make a script which will merge https://github.com/typeddjango/django-stubs/blob/master/django-stubs/shortcuts.pyi into the Django source, and then run mypy on top of it?
I think you should work on it before you make more changes to the stubs.

[kszmigiel]

@mkurnikov Thank you for precise task description, I'll take care of it as soon as possible 😃