title: python2.7: no such file or directory after brew upgrade python3
body: <p>After doing <code>brew update</code> and <code>brew upgrade python3</code>, I've been having such as <code>bad interpreter: /usr/local/opt/python/bin/python2.7: no such file or directory</code>. e.g.: </p>
  
  <pre><code>⇒  mkvirtualenv --python=/usr/local/bin/python3 labrador
  
  mkvirtualenv:78: /usr/local/bin/virtualenv: bad interpreter: /usr/local/opt/python/bin/python2.7: no such file or directory
  write() failed: Bad file descriptor
  </code></pre>