Simple case: switch to a Python with same major/minor version
-------------------------------------------------------------

>>> import sys
>>> print system(buildout),
Develop: '/sample-buildout/recipes'
>>> write(sample_buildout, 'buildout.cfg',
... """
... [buildout]
... develop = recipes
... extensions = slapos.rebootstrap
... python = installpython
...
... parts =
...   realrun
...
... [installpython]
... recipe = recipes:pyinstall
...
... [realrun]
... recipe = recipes:pyshow
... """ % dict(syspython=sys.executable))

>>> print system(buildout),
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>
Generated script '/sample-buildout/bin/buildout-rebootstrap'.
Develop: '/sample-buildout/recipes'
Updating installpython.
Installing realrun.
Running with: /sample_buildout/parts/installpython/bin/python

>>> print system(buildout),
Develop: '/sample-buildout/recipes'
Updating installpython.
Updating realrun.
Running with: /sample_buildout/parts/installpython/bin/python

>>> cp(buildout + '-orig', buildout)
>>> print system(buildout),
slapos.rebootstrap: Make sure that the section 'installpython' won't be reinstalled after rebootstrap.
Develop: '/sample-buildout/recipes'
Updating 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>
Develop: '/sample-buildout/recipes'
Updating installpython.
Updating realrun.
Running with: /sample_buildout/parts/installpython/bin/python
