title: Cellpose installation issue
body: <p>System: MacOS 10.14, python 3.8</p>
  <p>I get the following error when I try to pip  install Cellpose</p>
  <pre class="lang-none prettyprint-override"><code>ERROR: Could not build wheels for opencv-python-headless, which is required to install pyproject.toml-based projects.
  </code></pre>