title: git filter repo - Python was not found - but it's installed
body: <p>So I'm trying to use git filter-repo for the first time.
  I've installed Python 3.9.
  I try to run:
  <code>git filter-repo --strip-blobs-bigger-than 100M</code></p>
  <p>Each time it fails:</p>
  <ul>
  <li>Git Bash: <code>git: 'filter-repo' is not a git command.</code></li>
  <li>Powershell: <code>Python was not found;</code></li>
  <li>CMD: <code>Python was not found;</code></li>
  </ul>
  <p>Any advice on what I'm missing please?</p>