[deps] Update django-pipeline requirement in /images/openwisp_base

Updates the requirements on [django-pipeline](https://github.com/jazzband/django-pipeline) to permit the latest version.
- [Release notes](https://github.com/jazzband/django-pipeline/releases)
- [Changelog](https://github.com/jazzband/django-pipeline/blob/master/HISTORY.rst)
- [Commits](jazzband/django-pipeline@3.1.0...4.0.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

images/openwisp_base/requirements.txt

@@ -5,7 +5,7 @@ psycopg2
 sentry-sdk
 supervisor~=4.2  # allows 4.x and > 4.2
 django-cors-headers~=4.7
-django-pipeline~=3.1
+django-pipeline~=4.0
 uwsgi~=2.0.28
 django-celery-email~=3.0.0
 tldextract~=5.1.3