force_txt is deprecated after Django 3.

For django 4 : ImportError: cannot import name 'force_text' from 'django.utils.encoding' use forsce_str instead.

Checklist

[ ] I have verified that I am using a GIS-enabled database, such as PostGIS or Spatialite.
[ ] I have verified that that issue exists against the master branch of django-cities.
[ ] I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
[ ] I have reduced the issue to the simplest possible case.
[ ] I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)