Rebootstrap with a different major/minor version of Python
----------------------------------------------------------

TODO: The test is incomplete for 2 reasons:

- The test environment must be prepared with a directory containing the source
  tarballs of several eggs, at least `setuptools` and `zc.buildout`.
  Hence the failure that it couldn't find a distribution for `setuptools`.
- To go further, we'd need a really different version of Python instead of
  tricking `slapos.rebootstrap` that we have one, to reproduce the
  reinstallation of many eggs in `/eggs/` for the wanted version, and check
  that the change of signatures is ignored for the parts required to build
  Python.

>>> import sys
>>> write(sample_buildout, 'recipes', 'setup.py',
... """
... from setuptools import setup
...
... setup(
...   name = "recipes",
...   entry_points = {'zc.buildout':[
...     'pyinstall = pyinstall:Pyinstall',
...     'pyshow = pyshow:Pyshow',
...   ],
...   'zc.buildout.extension': ['ext = extension:extension'],},
...   )
... """)
>>> write(sample_buildout, 'recipes', 'extension.py',
... """
... import sys
... class extension(object):
...   version_info = ()
...   def __init__(self, buildout):
...     from slapos import rebootstrap
...     assert sys is rebootstrap.sys
...     rebootstrap.sys = self
...   def __getattr__(self, attr):
...     return getattr(sys, attr)
... """)
>>> print system(buildout),
Develop: '/sample-buildout/recipes'
>>> write(sample_buildout, 'buildout.cfg',
... """
... [buildout]
... develop = recipes
... extensions = slapos.rebootstrap recipes
... python = installpython
...
... parts =
...   realrun
...
... [installpython]
... recipe = recipes:pyinstall
...
... [realrun]
... recipe = recipes:pyshow
... """ % dict(syspython=sys.executable))

>>> print system(buildout), # doctest: +ELLIPSIS
slapos.rebootstrap: Make sure that the section 'installpython' won't be reinstalled after rebootstrap.
Develop: '/sample-buildout/recipes'
Installing installpython.
slapos.rebootstrap: 
************ REBOOTSTRAP: IMPORTANT NOTICE ************
bin/buildout is being reinstalled right now, as new python:
  /sample_buildout/parts/installpython/bin/python
is available, and buildout is using another python:
  /system_python
Buildout will be restarted automatically to have this change applied.
************ REBOOTSTRAP: IMPORTANT NOTICE ************
<BLANKLINE>
While:
  Installing.
Error: Couldn't find a distribution for 'setuptools==...'
