Update pytest-django requirement from ==3.6.* to ==3.7.*

Updates the requirements on [pytest-django](https://github.com/pytest-dev/pytest-django) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest-django/releases)
- [Changelog](https://github.com/pytest-dev/pytest-django/blob/master/docs/changelog.rst)
- [Commits](pytest-dev/pytest-django@v3.6.0...v3.7.0)

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

requirements/test.txt

@@ -1,6 +1,6 @@
 pytest==5.2.*
 pytest-cov==2.8.*
-pytest-django==3.6.*
+pytest-django==3.7.*
 pytest-pythonpath==0.7.*
 factory-boy==2.12.*
 mixer==6.1.*