Bump django from 2.2.13 to 2.2.18

Bumps [django](https://github.com/django/django) from 2.2.13 to 2.2.18.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.13...2.2.18)

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

Author: dependabot[bot]

requirements_dev.txt

@@ -4,7 +4,7 @@ pytest-flake8
 pytest-cov==2.5.1
 falcon==2.0.0
 flask
-django==2.2.13; python_version>="3.0"
+django==2.2.18; python_version>="3.0"
 requests==2.22.0
 responses==0.10.12
 webob