sslutil: add docstring to wrapsocket() Security should not be opaque.

sslutil: remove indentation in wrapsocket declaration It is no longer needed because we have a single code path.

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.

sslutil: store OP_NO_SSL* constants in module scope An upcoming patch will introduce a global SSLContext type so we have a single function used to wrap sockets. Prepare for that by introducing module level constants for disabling SSLv2 and SSLv3.

sslutil: better document state of security/ssl module Pythons older than 2.7.9 are lacking the modern ssl module and have horrible security. Let's document this explicitly.

tests: make tinyproxy.py use print_function

run-tests: use canonpath for with-python3

run-tests: add canonpath function consistently use realpath+expanduser