title: Error while pushing to Heroku: requested runtime is not available for this stack
body: <p>After creating a repository in Heroku, I want to push my project from master branch to my heroku repo. </p>
  
  <p>On my first try I got the following error message:</p>
  
  <blockquote>
    <p>No default language could be detected for this app</p>
  </blockquote>
  
  <p>After searching, I used the following command:</p>
  
  <pre><code>heroku buildpacks:set heroku/python
  </code></pre>
  
  <p>Now it gave me this error message:</p>
  
  <blockquote>
    <p>App not compatible with buildpack:
    <a href="https://buildpack-registry.s3.amazonaws.com/buildpacks/heroku/python.tgz" rel="noreferrer">https://buildpack-registry.s3.amazonaws.com/buildpacks/heroku/python.tgz</a></p>
  </blockquote>
  
  <p>So now I tried everything from the beginning and it gives me the following error:</p>
  
  <blockquote>
    <p>Python app detected</p>
    
    <p>Requested runtime ("python-3.7.1") is not
    available for this stack (heroku-18).</p>
  </blockquote>
  
  <p>I am stuck here.</p>
  
  <p>I do have a <code>requirements.txt</code> with all the requirements and a <code>runtime.txt</code> in the same folder as my <code>manage.py</code> file.</p>