title: How to avoid installing requirements.txt before every stage in .gitlab-ci.yml?
body: <p>I have a <code>.gitlab-ci.yml</code> which looks like this:</p>
  
  <pre><code>image: "python:3.7"
  
  before_script:
    - pip install -r requirements.txt
  
  stages:
    - stageA
    - stageB
  
  stage_a:
    stage: stageA
    script:
    - run_some_python_scripts
  
  stage_b:
    stage: stageB
    script:
    - run_more_python_scripts
  </code></pre>
  
  <p>With this setup, <code>requirements.txt</code> is installed before every stage.
  I need it installed only once, such that both <code>stageA</code> and <code>stageB</code> can use.</p>
  
  <p>How can I achieve this?</p>