Unused dirs in global site-packages directory.

After installing `drfdocs` there are unused directories in `site-packages`, such as `site` and `tests`.
In `setup.py` file param `packages` should contain only `"rest_framework_docs"` entry, something like this:

```
...
packages=['rest_framework_docs', ],
...
```

It is important also because unused directories have very common names (`site` and `tests`).

Open setup.py and inspect the packages parameter, which currently causes site and tests directories to be installed. Limit the package list to rest_framework_docs as described, then install drfdocs in a clean environment and verify that only the intended package directory is present.

Written by the indexing model from the issue text.