python: 2.7.9rc1 devel

ensurepip is backported to 2.7.9; use that to get pip instead of staging pip and setuptools ourselves.
Yeah, we can work around some of these. There's discussion of objection 2 upstream; I was invited to chime in on pypa/pip#1397 on IRC today.

Another complication unique to Homebrew that just occurred to me, call it 4): If we use ensurepip, we want to do something to remove the /Library site-packages from sys.path, which we add in sitecustomize.py, because there might be a system pip there. Ignoring sitecustomize.py with `python -S` is not a good solution because pip's shebang is derived from sys.executable, which gets set to the opt path in sitecustomize.py. This is reasonably fatal for python2 but irrelevant for python3 (for now). (I'm deeply ambivalent about adding the /Library site-packages to our sys.path anyway; maybe it's worth revisiting now.)

>  I do think it'd be good to sync up the approaches between python and python3 but that can be another PR.

Yeah, I agree.

While we're talking about it, do we need all the stdenv code in these formulae? I don't understand why someone would build Python without superenv.