title: PEP 517 method to specify build requirements is not working in poetry
body: <p>How can I specify the &quot;build system&quot; requirements in poetry? I think <a href="https://peps.python.org/pep-0517/" rel="nofollow noreferrer">PEP517</a> deals with this, but I can't get it to work.</p>
  <p>I am using poetry to manage my project. One of my project's dependencies is built from source, and building the dependency fails because the tools needed during the build are not available.</p>
  <p>The <a href="https://github.com/python-poetry/poetry/issues/3364" rel="nofollow noreferrer">dependency</a> has as <code>setup.py</code>, but no <code>setup.cfg</code> or <code>pyproject.toml</code>. Following the discussion in <a href="https://github.com/python-poetry/poetry/issues/3364" rel="nofollow noreferrer">this poetry issue</a>, I have tried adding:</p>
  <pre><code>[build-system]
  build-backend = 'setuptools.build_meta'
  requires = [
      &quot;packaging&quot;,
      &quot;Cython&quot;,
      &quot;setuptools&quot;,
  ]
  </code></pre>
  <p>to:</p>
  <ul>
  <li>the <code>pyproject.toml</code> file in my project (where the <code>poetry install</code> is initiated)</li>
  <li>a new <code>pyproject.toml</code> file in dependency's source tree.</li>
  </ul>
  <p><em>i.e.</em> building the dependency requires <code>Cython</code>, <code>packaging</code> and <code>setuptools</code>, as you can see from the first few lines in the dependency's <a href="https://github.com/NVIDIA/cuQuantum/blob/main/python/setup.py" rel="nofollow noreferrer">setup.py</a>.</p>
  <p>Adding the <code>[build-system]</code> section doesn't seem to do anything The build appears to be done in a temporary virtual env, and that env does not have the required tools (<code>Cython</code>, <code>packaging</code> and <code>setuptools</code>).</p>
  <p>BTW - I don't know if the <code>build-backend = 'setuptools.build_meta'</code> is correct (I think that means <code>setuptools</code> is being used to handle the dependency packaging, but I don't really know).</p>
  <p>What do I need to do to fix this?</p>