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.
Hi Julian, you are absolutely right, but a) this was present in the original code (which doesn't make it right), and b) min_cython_version is None anyways, which probably means this comparison should fail for Python 3?

I can fix this as well, but I'd appreciate a pointer to the best practice re. comparing versions for packages that only provide a version string. You certainly have more practical experience than I do. My first thought was to parse this into a tuple by splitting on "." and mapping an int() to it, but then Cython sometimes has versions like 0.20.2rc1...