There is a small mistake in the docs:

def bit_length(x):
    'Number of bits necessary to represent self in binary.'
    s = bin(x)          # binary representation:  bin(-37) --> '-0b100101'
    s = s.lstrip('-0b') # remove leading zeros and minus sign
    return len(s)       # len('100101') --> 6

is probably supposed to be:

def bit_length(x):
    'Number of bits necessary to represent x in binary.'
    s = bin(x)          # binary representation:  bin(-37) --> '-0b100101'
    s = s.lstrip('-0b') # remove leading zeros and minus sign
    return len(s)       # len('100101') --> 6