Build(deps): Bump djangorestframework from 3.11.2 to 3.15.2

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.11.2 to 3.15.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](https://github.com/encode/django-rest-framework/compare/3.11.2...3.15.2)

---
updated-dependencies:
- dependency-name: djangorestframework
  dependency-type: direct:production
...

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

requirements/requirements.txt

@@ -1,7 +1,7 @@
 django==2.2.28
 django-haystack==3.1.1
 django-filter==2.4.0
-djangorestframework==3.12.4
+djangorestframework==3.15.2
 dj-database-url==0.5.0
 django-braces==1.14.0
 django-crispy-forms==1.7.2

setup.py

@@ -12,7 +12,7 @@ install_requires = [
     'django>=1.11.19,<3.0',
     'django-haystack==2.8.1',
     'django-filter==2.0.0',
-    'djangorestframework==3.11.2',
+    'djangorestframework==3.15.2',
     'dj-database-url==0.5.0',
     'django-braces==1.9.0',
     'django-crispy-forms==1.7.2',