Pin djangorestframework to 3.15.1

Right now djangorestframework to 3.15.2 ships with one dir that was intent to be created only by the documentation example:

├── djangorestframework.egg-info
├── LICENSE.md
├── MANIFEST.in
├── PKG-INFO
├── README.md
├── rest_framework
├── setup.cfg
├── setup.py
├── tests
└── tutorial

 tree tutorial/
tutorial/
├── asgi.py
├── __init__.py
├── quickstart
│   ├── admin.py
│   ├── apps.py
│   ├── __init__.py
│   ├── migrations
│   │   └── __init__.py
│   ├── models.py
│   ├── serializers.py
│   ├── tests.py
│   └── views.py
├── settings.py
├── urls.py
└── wsgi.py

In the RPM side of packaging this is easy to solve, we can just add the macro %exclude , for users installing from pypi this might be a problem. We caught this here https://github.com/theforeman/pulpcore-packaging/pull/1323

Read requirements.txt at the referenced line and review the reported djangorestframework 3.15.2 package contents. Update the dependency to 3.15.1, then verify that the package version no longer includes the unintended tutorial directory for PyPI users.