[deps] Update freezegun requirement from ~=1.1.0 to ~=1.5.1

Updates the requirements on [freezegun](https://github.com/spulec/freezegun) to permit the latest version.
- [Release notes](https://github.com/spulec/freezegun/releases)
- [Changelog](https://github.com/spulec/freezegun/blob/master/CHANGELOG)
- [Commits](spulec/freezegun@1.1.0...1.5.1)

---
updated-dependencies:
- dependency-name: freezegun
  dependency-type: direct:production
...

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

requirements-test.txt

@@ -3,7 +3,7 @@ djangorestframework-simplejwt
 django-cors-headers>=2.5.2
 packaging
 openwisp-sphinx-theme~=1.0.2
-freezegun~=1.1.0
+freezegun~=1.5.1
 django-extensions
 openwisp-utils[qa] @ https://github.com/openwisp/openwisp-utils/tarball/master
 pylinkvalidator