Removed the python version checking.

As discussed on the mailinglist: better to have python version checking
as a separate buildout option instead of hidden in the [versions] part.
And... the python version checking itself needs to be more robust.
sys.version is too long, minor/major needs explicit checking.

CHANGES.rst

@@ -16,11 +16,6 @@ Change History
   - If ``update-versions-file`` is set to a filename (relative to the buildout
     directory), the ``show-picked-versions`` output is appended to that file.
 
-- Also from buildout-versions: if a ``python`` version is given in the
-  versions, buildout now checks if it is running with that python
-  version. Handy if you know if the buildout only functions with a specific
-  python version.
-
 
 2.0.0b1 (2013-01-21)
 ====================

src/zc/buildout/buildout.py

@@ -233,13 +233,6 @@ class Buildout(DictMixin):
                  if k not in versions
                  ))
 
-        python_version = versions.get('python')
-        if python_version and python_version[0] not in sys.version:
-            raise zc.buildout.easy_install.IncompatibleConstraintError(
-                "python version specified not found in sys.version:",
-                python_version
-                )
-
         self._annotated = copy.deepcopy(data)
         self._raw = _unannotate(data)
         self._data = {}

src/zc/buildout/repeatable.txt

@@ -390,51 +390,3 @@ configured.
     Error: The extension buildout-versions is now included in buildout itself.
     Remove the extension from your configuration and look at the `show-picked-versions`
     option in buildout's documentation.
-
-
-Controlling the python version
--------------------------------
-
-Sometimes you know that your buildout should always use a specific python
-version. You can add use ``python`` in your version list and buildout will
-give an error if the version doesn't match:
-
-    >>> write('buildout.cfg',
-    ... '''
-    ... [buildout]
-    ... parts = foo
-    ...
-    ... [versions]
-    ... python = 1.5
-    ... spam = 1
-    ...
-    ... [foo]
-    ... recipe = spam
-    ... ''')
-
-If we run the buildout, it will complain:
-
-    >>> print_(system(buildout), end='')
-    While:
-      Initializing.
-    Error: python version specified not found in sys.version: ('1.5', '/sample-buildout/buildout.cfg')
-
-Now the same with a correct version:
-
-    >>> import sys
-    >>> write('buildout.cfg',
-    ... '''
-    ... [buildout]
-    ... parts = foo
-    ...
-    ... [versions]
-    ... python = %s.%s
-    ... spam = 1
-    ...
-    ... [foo]
-    ... recipe = spam
-    ... ''' % (sys.version_info.major, sys.version_info.minor))
-    >>> print_(system(buildout), end='')
-    Uninstalling foo.
-    Installing foo.
-    recipe v1