As specified in PEP 477, this backports PEP 453 (ensurepip) to the Python 2.7 branch. Key differences from PEP 453 are:

* It is not run by default in the Makefile
* There is no venv modules, so downstream can remove it (though are asked to patch it to provide instructions redirecting people to how they should install pip).
* The ``pip`` command is installed as well as ``pipX`` and ``pipX.Y``.
* Given the above, --default-pip is hidden and no-oped and --no-default-pip is added to restore the Python 3.x behavior.

This also includes a (slightly modified to make it work on 2.x) backport of unittest.mock as test._mock_backport in order to make it reasonable to run the ensurepip tests without actually installing pip.

This patch does not include any changes to the Windows installers or to the OS X installers. I've nosey'd Ned Deily for the OS X installer changes, I'm not sure who is doing the Windows Installers now adays.