title: Plone content type relation cycle
body: <p>I want two content type like:</p>
  
  <pre class="lang-python prettyprint-override"><code># in content_type_a.py
  from content_type_b import IContentTypeB
  class IContentTypeA(model.Schema):
      content_type_bs = RelationList(
          default=[],
          value_type=RelationChoice(
              source=ObjPathSourceBinder(
                  object_provides=IContentTypeB.__identifier__
              ),
          ),
          required=False,
      )
  
  # in content_type_b.py
  from content_type_a import IContentTypeA
  class IContentTypeB(model.Schema):
      content_type_as = RelationList(
          default=[],
          value_type=RelationChoice(
              source=ObjPathSourceBinder(
                  object_provides=IContentTypeA.__identifier__
              ),
          ),
          required=False,
      )
  </code></pre>
  
  <p>But have error: ImportError: cannot import name IContentTypeA</p>
  
  <p>any idea for me?
  Thank you so much!</p>