get foolscap to declare its dependency on pyOpenSSL

[zooko]

Per ​http://foolscap.lothar.com/trac/ticket/66 , if foolscap will declare its dependency on pyOpenSSL, then we can remove our declaration of our dependency on pyOpenSSL. This makes sense, for example if a future release of foolscap comes with a different SSL implementation or a completely different encryption protocol, Tahoe's dependency on pyOpenSSL will disappear, and the Tahoe setup.py itself should not need to know about that change.

If foolscap wants to preserve the possibility of a foolscap install which doesn't require pyOpenSSL and can't do secure mode, then it could declare the existence of a foolscap "extra" named "secure_connections" or something like that, and Tahoe could declare that it depends on "foolscap with secure_connections", like this:

install_requires.append("foolscap[secure_connections]")

[warner]

done, in yesterday's foolscap-0.2.8 release