title: extras_require with readthedocs and poetry
body: <p>I use poetry to build my package with cython extensions, so install package flag is enabled. I'd like to host documentation via readthedocs. Poetry uses <code>build.py</code> file to generate <code>setup.py</code> with <code>poetry build</code> command. However readthedocs does not support <code>build.py</code> so I provided <code>setup.py</code> with following contents:</p>
  
  <pre><code>from setuptools import setup
  from build import *
  
  global setup_kwargs
  
  setup_kwargs = {}
  
  build(setup_kwargs)
  setup(**setup_kwargs)
  </code></pre>
  
  <p>To get rid of <code>requirements.txt</code> file in docs folder, I would like to add <code>extras_require</code> parameter to setup, so the last line in <code>setup.py</code>:</p>
  
  <pre><code>setup(extras_require={'docs': ['toml']}, **setup_kwargs)
  </code></pre>
  
  <p>Readthedocs calls <code>/&lt;path&gt;/envs/latest/bin/python -m pip install --upgrade --upgrade-strategy eager --no-cache-dir .[docs]</code></p>
  
  <p><strong>WARNING:  does not provide the extra 'docs'</strong> and importing toml from <code>docs/conf.py</code> fails</p>
  
  <p>If I add extras to <code>pyproject.toml</code>:</p>
  
  <pre><code>[tool.poetry.extras]
  docs = ['toml']
  </code></pre>
  
  <p>Warning disappears but still rtd fails to import toml.</p>
  
  <p>My <code>.readthedocs.yml</code>:</p>
  
  <pre><code>version: 2
  
  sphinx:
    configuration: docs/conf.py
  
  formats: all
  
  python:
    version: 3.7
    install:
      - method: pip
        path: .
        extra_requirements:
          - docs
  
  submodules:
    include: all
  </code></pre>