title: Pyinstaller error: Unrecognized arguments when passing a spec file
body: <p>I run <code>pyi-makespec --onefile MyScript.py</code>.
  After this I add <code>a.datas += Tree('./MyData', prefix='.')</code>
  to the generated spec file. Then i tried to run:</p>
  
  <pre><code>pyinstaller MyScript.py --onefile MyScript.spec
  </code></pre>
  
  <p>but Pyinstaller returns with:</p>
  
  <pre><code>pyinstaller-script.py: error: unrecognized arguments: MyScript.spec
  </code></pre>
  
  <p>I'm using PyInstaller 3.3.1 and Python 3.6.5.</p>
  
  <p>Have anyone an idea why it didn't let me pass the spec file?</p>