[deps] Update django-sortedm2m requirement from ~=3.1.1 to ~=4.0.0

Updates the requirements on [django-sortedm2m](https://github.com/jazzband/django-sortedm2m) to permit the latest version.
- [Release notes](https://github.com/jazzband/django-sortedm2m/releases)
- [Changelog](https://github.com/jazzband/django-sortedm2m/blob/master/CHANGES.rst)
- [Commits](jazzband/django-sortedm2m@3.1.1...4.0.0)

---
updated-dependencies:
- dependency-name: django-sortedm2m
  dependency-type: direct:production
...

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

requirements.txt

@@ -1,4 +1,4 @@
-django-sortedm2m~=3.1.1
+django-sortedm2m~=4.0.0
 django-reversion~=5.1.0
 django-taggit~=4.0.0
 netjsonconfig @ https://github.com/openwisp/netjsonconfig/tarball/master