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.
Oh, I see, these come from  PyTables/tables/req_versions.py which is exec'ed in the current scope:

    min_cython_version = '0.13'