Fix missing __init__.py in django_stubs_ext.db

[monosans]

The django_stubs_ext 4.2.0 release does not have a db package because it is missing __init__.py.

[adamchainz]

@monosans what issues are you seeing? After PEP420, folders without __init__.py should still function as packages.

@intgr if this is an issue, let's add https://github.com/adamchainz/flake8-no-pep420 to flake8 configuration.

[intgr]

This PR references the issue described here: #1450 (comment)

I tested python setup.py sdist from this branch and it still produced an archive without db/models.py. Unless I messed something up.

I think something needs to be changed in django_stubs_ext/setup.py, but I haven't yet looked into it in depth.

[intgr]

My bad, this PR indeed fixes the issue, thanks!

├── django-stubs-ext-4.2.0
│   ├── PKG-INFO
│   ├── README.md
│   ├── django_stubs_ext
│   │   ├── __init__.py
│   │   ├── aliases.py
│   │   ├── annotations.py
│   │   ├── db
│   │   │   ├── __init__.py
│   │   │   └── models.py

[intgr]

Thanks for the help!