Python 3.6 underscores in numeric literal support

[achimnol]

Another new syntax in Python 3.6 is allowance of underscores in the middle of numeric literals to improve readability of long numbers. https://docs.python.org/3.6/whatsnew/3.6.html#whatsnew36-pep515

Let's add support for this as well. :)

[nfnty]

This has been fixed over at the fork vim-python/python-syntax.