Missing dependency: six

Therer's a missing dependency: six

With `drf-url-filters==0.5.1`, I had the following error:

```
0.800   File "/usr/local/lib/python3.14/site-packages/filters/mixins.py", line 1, in <module>
0.800     import six
0.800 ModuleNotFoundError: No module named 'six'
```

Adding the dependency `six==1.17.0` to `requirements.txt` resolved the issue. Please add this one to the package dependencies.