Handle missing pyopenssl dependency

[ryanwilsonperkin]

Alternative to #242
Fixes #241

Handle the case where pyopenssl is not installed and fall back to default of setting PyOpenSSLContext to None.

[altendky]

Maybe add test coverage with and without?

[ryanwilsonperkin]

@altendky not really sure how I'd go about doing that. Any recommendations?

[greatestape]

Could you test the fix by adding an additional tox build that omits pyopenssl?

[greatestape]

@ryanwilsonperkin I took a crack at adding test coverage of this. I tried rolling back your change to confirm that it would cause the test to fail, and it seems to cover it.

Let me know what you think.

ryanwilsonperkin#1

[ryanwilsonperkin]

Thanks @greatestape, added a couple of comments onto that PR. If you're available to respond now that'd be great, otherwise I'll accept as is and then pull your changes into this PR

[sigmavirus24]

Feel free to send the tests along in another PR. I'd like to get this released tonight

[nsoranzo]

Thanks @ryanwilsonperkin and @sigmavirus24!

[sigmavirus24]