Bump djangorestframework from 3.9.2 to 3.13.1

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.9.2 to 3.13.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.9.2...3.13.1)

---
updated-dependencies:
- dependency-name: djangorestframework
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

requirements.txt

@@ -21,7 +21,7 @@ django-cors-headers==3.10.1
 django-cryptography==1.0
 django-rest-swagger==2.2.0
 django-storages==1.11.1
-djangorestframework==3.9.2
+djangorestframework==3.13.1
 et-xmlfile==1.1.0
 flake8==4.0.1
 idna==3.3