[deps] Update django-model-utils requirement from ~=4.5.0 to >=4.5,<5.1

Updates the requirements on [django-model-utils](https://github.com/jazzband/django-model-utils) to permit the latest version.
- [Release notes](https://github.com/jazzband/django-model-utils/releases)
- [Changelog](https://github.com/jazzband/django-model-utils/blob/master/CHANGES.rst)
- [Commits](jazzband/django-model-utils@4.5.0...5.0.0)

---
updated-dependencies:
- dependency-name: django-model-utils
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Federico Capoano <f.capoano@openwisp.io>

Author: dependabot[bot]

setup.py

@@ -25,7 +25,7 @@
     scripts=['openwisp-qa-check', 'openwisp-qa-format', 'openwisp-pre-push-hook'],
     zip_safe=False,
     install_requires=[
-        'django-model-utils~=4.5.0',
+        'django-model-utils>=4.5,<5.1',
         'django-compress-staticfiles~=1.0.1b',
         'django-admin-autocomplete-filter~=0.7.1',
         'swapper~=1.3.0',