fix for binary requests response content in py3

The requests lib on python3 returns response.content as ```bytes``` type.  This is not accounted for in any of the current tests, thus incorrectly purporting python3 compatibility.

This PR includes:
1. A test (```test_get_200_json_py3```) that is identical to ```test_get_200_json``` except that the mock response content is binary data
2. A fix for ```_try_to_serialize_response``` that checks for a binary response and attempts to decode it
3. some ignores and a coverage report item in the tox config