Bump django from 2.2.3 to 2.2.8 in /docs/_ext/djangodummy

Bumps [django](https://github.com/django/django) from 2.2.3 to 2.2.8.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.3...2.2.8)

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

Author: dependabot[bot]

docs/_ext/djangodummy/requirements.txt

@@ -1,5 +1,5 @@
 # for readthedocs
-Django==2.2.3
+Django==2.2.8
 django-admin-tools==0.8.1
 dashboardmods==1.0
 sphinxcontrib-django==0.4