python: 2.7.9rc1 devel

ensurepip is backported to 2.7.9; use that to get pip instead of staging pip and setuptools ourselves.
A few reasons not to use it, probably in order of descending importance, are:

1) With ensurepip, we risk downgrading pip on post-install since pip and CPython have different release cycles and the bundled version ensurepip uses is only updated at each CPython maintenance release.

If we resource them ourselves we still force a version of pip and setputools at post-install but at least we get to bump those independent of a CPython release.

2) ensurepip can't be made to ignore user configuration in ~/.pydistutils.cfg. These are probably rare in practice but could lead to surprising outcomes.

3) We also need to make sure we have setuptools; ensurepip presently installs setuptools as a pip dependency (and that will work until at least the next CPython release) but we're supposed to consider that to be a private implementation detail that may change.

TLDR-y segue on why we have to remove old pips before using ensurepip:
If pip can be imported, ensurepip won't do anything; it doesn't check if the scripts exist in /usr/local/bin or not. The way we've installed pip before, when a user upgrades python, pip is importable after unlinking the old python but the scripts are gone. So, ironically, just using ensurepip leaves us without a "pip" command. So we have to forcibly remove any existing pip first.