Update django-taggit to 0.20.1

[pyup-bot]

There's a new version of django-taggit available.
You are currently using 0.19.1. I have updated it to 0.20.1

Changelog

0.20.1

* Specify `app_label` for `Tag` and `TaggedItem`
 * https://github.com/alex/django-taggit/pull/420



### 0.20.0

• Fix UnboundLocalError in _TaggableManager.set(..)
  • Fix UnboundLocalError in _TaggableManager.set(..) jazzband/django-taggit#418
• Update doc links to reflect RTD domain changes
  • Convert readthedocs links for their .org -> .io migration for hosted projects jazzband/django-taggit#417
• Improve Russian translations
  • Add ru translation of app verbose name jazzband/django-taggit#416

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖

[pyup-bot]

Closing this in favor of #113