* pin requests version in py26-oldest

* Determine requests security deps dynamically

Starting with requests 2.12, pyasn1 and ndg-httpsclient are no longer
needed to inject pyopenssl into urllib3. This change allows us to
determine whether or not these dependencies are required at install
time. If an older version of requests is used, these packages are
still installed. If a new version of requests is used, they are not
reducing the number of dependencies we have.

* Bump requests version in certbot-auto

* Use pkg_resources in activate test

Due to pip's lack of dependency resolution, the change to use
requests[extras] causes errors in acme.util_test because pkg_resources
accurately detects the "missing" dependency.

There isn't a real problem here. The problem comes from a brand new
requests and ancient pyopenssl as well as a unit test for
functionality we plan to remove in our next release. I modified
the unit test to fix the problem for now.

* Use six instead of pkg_resources for test

* Require requests<=2.11.1 in py27-oldest test

If we don't do this, we get test failures for the certbot package
which is actually a good thing! pkg_resources is catching the
unlikely but possible problem I describe in #3803 and erroring out
saying it is missing the necessary dependencies to run certbot.
Good job package resources.

* Undo changes to acme.util_test