sslutil: always use SSLContext Now that we have a fake SSLContext instance, we can unify the code paths for wrapping sockets to always use the SSLContext APIs. Because this is security code, I've retained the try..except to make the diff easier to read. It will be removed in the next patch. I took the liberty of updating the inline docs about supported protocols and how the constants work because this stuff is important and needs to be explicitly documented.

sslutil: move _canloaddefaultcerts logic We now have a newer block accessing SSLContext. Let's move this code to make subsequent refactorings of the former block easier.

sslutil: implement SSLContext class Python <2.7.9 doesn't have a ssl.SSLContext class. In this patch, we implement the interface to the class so we can have a unified code path for all supported versions of Python. This is similar to the approach that urllib3 takes.