Pytest-httpbin

* fix redirect Location to have "https://" scheme (thanks immerrr)
* Include regression tests in PyPI tarball by (thanks kmosiejczuk)

Update included self-signed cert to include IP address in SAN (See 52). Full version bump because this could be a breaking change for those depending on the certificate missing the IP address in the SAN (as it seems the requests test suite does)
Only use pytest.fixture decorator once (thanks hroncok)
Fix a few README typos (thanks hemberger)

* Allow to run httpbin on fixed port using environment variables (thanks hroncok)
* Allow server to be thread.join()ed (thanks graingert)
* Add support for Python 3.6 (thanks graingert)