[Python-3000] Allowing underscores in numeric constants.

[Ian D. Bollinger]

Personally I like the idea (stolen from Perl, and others) of allowing 
underscores in numbers as to group digits, for instance:
one_million = 1_000_000
maximum = 0xFF_FF_FF_FF

Once four or more identical digits appear in a row, I find a number 
starts to become unwieldy, and separators would be a useful visual aid.

-- Ian D. Bollinger