Bump django from 3.0.1 to 3.1.6

Bumps [django](https://github.com/django/django) from 3.0.1 to 3.1.6.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/3.0.1...3.1.6)

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

requirements.txt

@@ -1,6 +1,6 @@
 asgiref==3.2.3
 -e git+https://github.com/DataDog/dd-trace-py@7a45ee7229b3ffeaa06c3818289afeec9cb00034#egg=ddtrace
-Django==3.0.1
+Django==3.1.6
 djangorestframework==3.11.0
 psutil==5.6.7
 pytz==2019.3