Don't allow underscores when doing base conversion

[dloscutoff]

It turns out that Python 3 allows underscores in strings representing base-N numbers: int("a_b_c", 16) results in 2748, the same as int("abc", 16) does. This is undesirable behavior for Pip; underscores in the left argument of FB should give a result of nil, just like any other character that isn't a digit in the given base. I think the implementation of FROMBASE will have to check for underscores explicitly.