avoid "unused import" errors in __init__.py files

Often something like..
```
from mymodule.mysubmodule import some_function
```
in `.../mymodule/__init__.py` is used in order to enable the following usage:
```
>>> from mymodule import some_function
```
When using pyflakes (e.g. via flake8) often `__init__.py` files get checked, reporting errors that are not real errors but intended code. This can be avoided by using comment directives like `# NOQA` but those clutter the code base unnecessarily. So, I think `__init__.py` files should not be check for unused imports in general.