title: How to read file's content from GitHub file with a specific branch name?
body: <p>In branch name, <code>qa03</code>, in my repository, <code>repo1</code>, I have a file, <code>environment.yml</code>. </p>
  
  <p>The content of <code>environment.yml</code> is following:</p>
  
  <pre><code># This is chef config file
  
  chef:
    - xxx/chef
    - xxx/faster-git-clone
  bam:
    - xxx/asd
    - V20180820_902
  </code></pre>
  
  <p><strong>I want to read this whole file from my python script and store it into my string</strong> (so that I can send it to my Trello Card).</p>
  
  <p>Q: How do I read the content of the file from GitHub repo with specific branchname? (below is my pseudocode)</p>
  
  <pre><code>....
  # Read `environmet.yml from repo1/qa03`, and send it to Trello.
  string_content = read('repo1:qa03/environment.yml') #Q: how???
  send(string_content)
  </code></pre>