title: readthedocs build failing using aiohttp
body: <p>First time using readthedocs.</p>
  
  <p>Firstly, is there a way to specify build with python 3.6?</p>
  
  <p>My lastest build is here:
  <a href="https://readthedocs.org/projects/cfanalytics/builds/6772697/" rel="nofollow noreferrer">https://readthedocs.org/projects/cfanalytics/builds/6772697/</a>
  which uses python 2.7 for the build, but my project requires python >= 3.6.</p>
  
  <p>Secondly,
  It's failing because of an 'invalid syntax' at L125 <a href="https://github.com/raybellwaves/cfanalytics/blob/94aa21c628678f0cacf0006a92bb3740d51cca14/cfanalytics/core/cfopendata.py#L125" rel="nofollow noreferrer">https://github.com/raybellwaves/cfanalytics/blob/94aa21c628678f0cacf0006a92bb3740d51cca14/cfanalytics/core/cfopendata.py#L125</a>. However, the function is an Asynchronous function hence the <code>async def ...</code> I think <code>aiohttp</code> first appeared in python 3.5 so it may be an issue with the python version?</p>