title: Push rejected failed to compile python app
body: <p>so i get this error when i try to deploy my master branch, i have set my requirements.txt, runtime.txt and Procfile but i still get this error</p>
  <pre><code>-----&gt; Building on the Heroku-20 stack
  -----&gt; Using buildpack: heroku/python
  -----&gt; Python app detected
  -----&gt; Using Python version specified in runtime.txt
   !     Requested runtime (python - 3.9.7) is not available for this stack (heroku-20).
   !     Aborting.  More info: https://devcenter.heroku.com/articles/python-support
   !     Push rejected, failed to compile Python app.
   !     Push failed
  </code></pre>
  <p>my requirements.txt file</p>
  <pre><code>asgiref==3.4.1
  dj-database-url==0.5.0
  Django==3.2.7
  django-heroku==0.3.1
  gunicorn==20.1.0
  Pillow==8.4.0
  psycopg2==2.9.2
  python-decouple==3.5
  pytz==2021.3
  sqlparse==0.4.2
  whitenoise==5.3.0
  </code></pre>
  <p>please help</p>