[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 @ https://github.com/openwisp/netjsonconfig/tarball/1.2
 django-x509 @ https://github.com/openwisp/django-x509/tarball/1.3
 django-loci @ https://github.com/openwisp/django-loci/tarball/1.2
-django-flat-json-widget~=0.3.0
+django-flat-json-widget~=0.3.1
 openwisp-users @ https://github.com/openwisp/openwisp-users/tarball/1.2
 openwisp-utils[celery] @ https://github.com/openwisp/openwisp-utils/tarball/1.2
 openwisp-notifications @ https://github.com/openwisp/openwisp-notifications/tarball/1.2