[Cython] PEP 515: underscores in number literals

[Stefan Behnel]

Hi!

PEP 515 proposes to allow underscores in number literals to provide a
visual digit separator, e.g. for thousands, nibbles, hex-bytes/words, etc.

https://www.python.org/dev/peps/pep-0515/

I've implemented it for Cython:

https://github.com/cython/cython/pull/499

I think it's safe enough to still go into 0.24 as it only extends the
supported syntax, but I'll leave it to Robert to decide.

Stefan