Bump django from 3.1.2 to 3.1.8

Bumps [django](https://github.com/django/django) from 3.1.2 to 3.1.8.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/3.1.2...3.1.8

)

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