Require pyopenssl and add request backoff

[sbscully]

I have added a requirement for pyopenssl to the setup.py. The version is set to >=0.15.1 because that is the latest version that works with python 2.6 and 3.2 mentioned in the setup.py classifiers. pyopenssl 0.15.1 is quite old (April 2015) but I don't think that will be a problem.

I tried installing the pyopenssl pip package in a plain docker container without OpenSSL installed and it failed as expected, so I think it's fair to assume this is all that's needed to ensure OpenSSL is present.

[sbscully]

I've bumped a minor version and added to the changelog. I think this should be OK for merging now. Edit: I'll just add the backoff changes to this pull request.

[sbscully]

@mtmail I've added exponential backoff with jitter, max retries 5 and max time 120s. Only issue now is that this makes the tests take 20x longer to run, will fix that now. I couldn't think of a good way to test this directly, any ideas?

The failing test run is for pypy only, it is complaining about an out of date OpenSSL, not sure if I introduced that failure or it was already failing?

[sbscully]

@mtmail the tests only take a couple of seconds now. I also added generic requests.exceptions.RequestException exceptions to the backoff behaviour.

I think this is ready to merge now, let me know if you want me to make any changes.

[sbscully]

@mtmail Changing to bionic fixed the OpenSSL issue on travis, but it broke python 2.7 and pypy3 still didn't pass. I've reverted to the original config with trusty/xenial.