Update dependency django-webtest to v1.9.13

Author: renovate[bot]

requirements.txt

@@ -16,7 +16,7 @@ django-common-helpers==0.6.2
 django-concurrency==0.8.1
 django-cron==0.3.4
 django-filter==2.4.0
-django-webtest==1.7.7
+django-webtest==1.9.13
 djangorestframework>=3.9.1
 docutils==0.11
 kombu==3.0.23