title: Unclosed client session warning of unknown origin
body: <p>I'm using a third-party API client library that is based on aiohttp. The library doesn't implement it's clients as context managers, but that should be no problem as long as I always close the <code>aiohttp.ClientSession</code> manually (right?).</p>
  
  <p>The problem is that even after carefully doing this I still get the <code>Unclosed client session</code> warning without any information from where it is coming from. I'm capturing the warnings traceback with <code>pytest -W error::ResourceWarning</code>, but the result is simply</p>
  
  <pre><code>.Exception ignored in: &lt;bound method ClientSession.__del__ of &lt;aiohttp.client.ClientSession object at 0x10fc15ba8&gt;&gt;
  Traceback (most recent call last):
    File "&lt;PROJECT&gt;/venv/lib/python3.6/site-packages/aiohttp/client.py", line 211, in __del__
      **kwargs)
  ResourceWarning: Unclosed client session &lt;aiohttp.client.ClientSession object at 0x10fc15ba8&gt;
  </code></pre>
  
  <p>i.e. a traceback with a single frame.</p>
  
  <p>Can someone tell me what I'm doing wrong here and how I can get more information about that warning?</p>