Stop worrying about backwards-compatibility

[pquentin]

There are a number of things that were done in the urllib3 codebase that were done in the name of backwards-compatibility that can be removed now if that makes sense. This is an umbrella issue to record the small things that can be removed.

There's nothing listed right now, but grep for "backwards" or "deprecat" to find candidates.

[sethmlarson]

Removed pyOpenSSL and [secure] extra