Bump django from 2.2.6 to 2.2.8

Bumps [django](https://github.com/django/django) from 2.2.6 to 2.2.8.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.6...2.2.8)

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

Author: dependabot[bot]

requirements_dev.txt

@@ -3,4 +3,4 @@ pytest==3.5.0
 pytest-flake8
 pytest-cov==2.5.1
 flask
-django==2.2.6; python_version>="3.0"
+django==2.2.8; python_version>="3.0"