title: Requested runtime (Python-3.7.3) is not available for this stack (heroku-18)
body: <p>I'm using Django 2.2.3 and I'm getting this error:</p>
  
  <pre><code>Requested runtime (Python-3.7.3) is not available for this stack (heroku-18)
  </code></pre>
  
  <p>But, <a href="https://devcenter.heroku.com/articles/python-support" rel="nofollow noreferrer">Heroku's Python documentation</a> says that <code>python-3.7.3</code> should be available in stack <code>heroku-18</code>.</p>
  
  <p><code>runtime.txt</code> contains <code>Python-3.7.3</code>. </p>
  
  <p><strong>FULL ERROR</strong></p>
  
  <pre><code>Counting objects: 100% (43/43), done.
  Delta compression using up to 4 threads
  Compressing objects: 100% (36/36), done.
  Writing objects: 100% (43/43), 10.70 KiB | 996.00 KiB/s, done.
  Total 43 (delta 6), reused 0 (delta 0)
  remote: Compressing source files... done.
  remote: Building source:
  remote:
  remote: -----&gt; Python app detected
  remote:  !     Requested runtime (Python-3.7.3) is not available for this stack (heroku-18).
  remote:  !     Aborting.  More info: https://devcenter.heroku.com/articles/python-support
  remote:  !     Push rejected, failed to compile Python app.
  remote:
  remote:  !     Push failed
  </code></pre>