add stub for core/asgi.py from Django 3

[gregbaker]

When checking a Django 3 project, I get this error (with sufficient strictness):

$ mypy .
foo/asgi.py:12: error: No library stub file for module 'django.core.asgi'
foo/asgi.py:12: note: (Stub files are from https://github.com/python/typeshed)
Found 1 error in 1 file (checked 15 source files)

The PR creates a stub for that file, which is new in Django 3.

[sobolevn]

Thanks! We are currently working on different version support in django-stubs. It would be an awesome addition!

[askoretskiy]

I there any way I could contribute here? I face the same error with new Django 3.0 project.

[palfrey]

FYI I also needed a stub ASGIHandler to make this work

[palfrey]

This appears to have been fixed in #527