[3.0.x] Fixed #30944 -- Changed reusable apps docs to use a declarative config.

Backport of 89368ab6e3 from master

docs/intro/reusable-apps.txt

@@ -181,50 +181,51 @@ this. For a small app like polls, this process isn't too difficult.
    license. Just be aware that your licensing choice will affect who is able
    to use your code.
 
-#. Next we'll create a ``setup.py`` file which provides details about how to
-   build and install the app.  A full explanation of this file is beyond the
-   scope of this tutorial, but the `setuptools docs
-   <https://setuptools.readthedocs.io/en/latest/>`_ have a good
-   explanation. Create a file ``django-polls/setup.py`` with the following
-   contents:
+#. Next we'll create ``setup.cfg`` and ``setup.py`` files which detail how to
+   build and install the app. A full explanation of these files is beyond the
+   scope of this tutorial, but the `setuptools documentation
+   <https://setuptools.readthedocs.io/en/latest/>`_ has a good explanation.
+   Create the files ``django-polls/setup.cfg`` and ``django-polls/setup.py``
+   with the following contents:
+
+   .. code-block:: ini
+        :caption: django-polls/setup.cfg
+
+        [metadata]
+        name = django-polls
+        version = 0.1
+        description = A Django app to conduct Web-based polls.
+        long_description = file: README.rst
+        url = https://www.example.com/
+        author = Your Name
+        author_email = yourname@example.com
+        license = BSD-3-Clause  # Example license
+        classifiers =
+            Environment :: Web Environment
+            Framework :: Django
+            Framework :: Django :: X.Y  # Replace "X.Y" as appropriate
+            Intended Audience :: Developers
+            License :: OSI Approved :: BSD License
+            Operating System :: OS Independent
+            Programming Language :: Python
+            Programming Language :: Python :: 3
+            Programming Language :: Python :: 3 :: Only
+            Programming Language :: Python :: 3.6
+            Programming Language :: Python :: 3.7
+            Programming Language :: Python :: 3.8
+            Topic :: Internet :: WWW/HTTP
+            Topic :: Internet :: WWW/HTTP :: Dynamic Content
+
+        [options]
+        include_package_data = true
+        packages = find:
 
    .. code-block:: python
         :caption: django-polls/setup.py
 
-       import os
-       from setuptools import find_packages, setup
+        from setuptools import setup
 
-       with open(os.path.join(os.path.dirname(__file__), 'README.rst')) as readme:
-           README = readme.read()
-
-       # allow setup.py to be run from any path
-       os.chdir(os.path.normpath(os.path.join(os.path.abspath(__file__), os.pardir)))
-
-       setup(
-           name='django-polls',
-           version='0.1',
-           packages=find_packages(),
-           include_package_data=True,
-           license='BSD License',  # example license
-           description='A Django app to conduct Web-based polls.',
-           long_description=README,
-           url='https://www.example.com/',
-           author='Your Name',
-           author_email='yourname@example.com',
-           classifiers=[
-               'Environment :: Web Environment',
-               'Framework :: Django',
-               'Framework :: Django :: X.Y',  # replace "X.Y" as appropriate
-               'Intended Audience :: Developers',
-               'License :: OSI Approved :: BSD License',  # example license
-               'Operating System :: OS Independent',
-               'Programming Language :: Python',
-               'Programming Language :: Python :: 3.6',
-               'Programming Language :: Python :: 3.7',
-               'Topic :: Internet :: WWW/HTTP',
-               'Topic :: Internet :: WWW/HTTP :: Dynamic Content',
-           ],
-       )
+        setup()
 
 #. Only Python modules and packages are included in the package by default. To
    include additional files, we'll need to create a ``MANIFEST.in`` file. The