AstroResponse inherit from requests.Response

@agroener - I think this will solve your problems

@jwoillez - is there any good reason *not* to do this?  Besides the test failures... those are probably a good reason.  But, can we chat tomorrow?  I'm not sure if `Response` objects are pickleable, but if they're not, we need to find the maximally-compatible class that can still be pickled.