Update PyTables to use Cython.__version__

Suggested changes according to the recommendation from Cython developers:

http://article.gmane.org/gmane.comp.python.cython.user/11768

Thanks for considering.
there is no good way to deal with it, distutils.Strict/LooseVersion sometimes works, but usually not with rc/dev/a/b versioning used in the scipy environment, but it could be good enough depending on what you want to check.
just converting the first two digits to int might also be good enough, depending on how fixed two cythons major.minor.patch versioning scheme is.

Numpy added a `NumpyVersion` class to allow users to deal with its versioning scheme. Maybe cython has something similar?