build(deps): bump django in /testing/plugins_integration

Bumps [django](https://github.com/django/django) from 3.1.2 to 3.1.3.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/3.1.2...3.1.3)

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

testing/plugins_integration/requirements.txt

@@ -1,5 +1,5 @@
 anyio[curio,trio]==2.0.2
-django==3.1.2
+django==3.1.3
 pytest-asyncio==0.14.0
 pytest-bdd==4.0.1
 pytest-cov==2.10.1