[deps] Update django-flat-json-widget requirement

Updates the requirements on [django-flat-json-widget](https://github.com/openwisp/django-flat-json-widget) to permit the latest version.
- [Release notes](https://github.com/openwisp/django-flat-json-widget/releases)
- [Changelog](https://github.com/openwisp/django-flat-json-widget/blob/master/CHANGES.rst)
- [Commits](openwisp/django-flat-json-widget@0.3.0...0.3.1)

---
updated-dependencies:
- dependency-name: django-flat-json-widget
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -4,7 +4,7 @@ django-taggit~=4.0.0
 netjsonconfig~=1.1.0
 django-x509~=1.2.0
 django-loci~=1.1.0
-django-flat-json-widget~=0.3.0
+django-flat-json-widget~=0.3.1
 openwisp-users~=1.1.0
 openwisp-utils[celery]~=1.1.1
 openwisp-notifications~=1.1.0