Bump django from 3.1.7 to 3.1.8 in /testing/plugins_integration

Bumps [django](https://github.com/django/django) from 3.1.7 to 3.1.8.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/3.1.7...3.1.8)

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

testing/plugins_integration/requirements.txt

@@ -1,5 +1,5 @@
 anyio[curio,trio]==2.2.0
-django==3.1.7
+django==3.1.8
 pytest-asyncio==0.14.0
 pytest-bdd==4.0.2
 pytest-cov==2.11.1