aiohttp adapter

[kylebarron]

We should have an adapter in the Python module to enable usage of aiohttp, and not just obstore as the request mechanism here.

• Check if the input object is an instance of aiohttp.ClientSession
• Connect to AsyncFileReader trait. Should be just specific calls into get

cc @geospatial-jeff

[kylebarron]

Now that we've merged #64, any "store" conforming to obspec.GetRangeAsync and GetRangesAsync will work, so we can easily have a Python-side wrapper to support aiohttp (and others) in the future if we'd like.