[Distutils] Classifiers for Python versions

["Martin v. Löwis"]

As suggested in issue 2169549, I have now added a number of
trove classifiers to PyPI to denote support for certain Python
versions, namely

Programming Language :: Python :: 2
Programming Language :: Python :: 2.3
Programming Language :: Python :: 2.4
Programming Language :: Python :: 2.5
Programming Language :: Python :: 2.6
Programming Language :: Python :: 2.7
Programming Language :: Python :: 3
Programming Language :: Python :: 3.0
Programming Language :: Python :: 3.1

With these, packages can indicate that they support 2.x
(but not 3.x), or that they have been tested/written for
specific Python releases.

As a side effect, packages can now expressly state that they
support Python 3000.

Regards,
Martin