Mypy complains about adrf.APIView  when running in strict mode

Running mypy with the stric flag:
```bash
mypy views --check-untyped-defs --strict
```

Caused several errors of the kind:
```bash
views.py:48: error: Class cannot subclass "APIView" (has type "Any")  [misc]
```

Would it be possible to improve the typing of the package to include the `py.typed` file and these little details?