title: pip3 install ruamel.yaml[jinja2] breaks ruamel.yaml
body: <p>I have a docker container that has been using ruamel.yaml and ruamel.yaml.jinja2 for a while. Lately it has begun to fail. The following dockerfile reproduces the problem:</p>
  
  <pre><code>FROM ubuntu
  
  RUN apt update; apt install -y python3 python3-pip
  RUN pip3 install -U pip setuptools wheel
  RUN pip install ruamel.yaml
  RUN python3 -c "from ruamel.yaml import YAML"
  
  RUN pip install ruamel.yaml[jinja2]
  RUN python3 -c "from ruamel.yaml import YAML"
  </code></pre>
  
  <p>The first python3 command succeeds, but the last fails with the following message: ImportError: cannot import name 'YAML'</p>
  
  <p>I have tried specifying earlier versions of ruamel.yaml, but with no success, unless I go all the way back to 0.15.0 (from 2017). </p>
  
  <p>I have also tried different versions of python3.x and pip with no change.</p>