Accept `_` in number litterals

[donnerc]

The Python grammar allows underscores in the number litterals to increase readability. For instance, Python accepts litterals like 1_000_000 for 1 million. It would be great if the parser would accept that syntax.

[Tobias-Kohn]

You are right; this should have been included already a while back. I will fix this as soon as possible.