Convert longs to / from bytes, mimicking behavior of Python 3.6 int.from/to_bytes

bint.py

def bytes2int(data, byteorder='little', signed=False):
    """ replicate int.from_bytes from python3 """
    if byteorder == 'little':
        data = reversed(data)
    elif byteorder != 'big':
        raise ValueError("byteorder must be either 'little' or 'big'")
    bl = bytearray(data)

    val = 0L
    for b in bl:
        val <<= 8
        val += b

    if signed:
        bl = (val.bit_length() + 7) // 8 * 8
        if val & (1 << (bl - 1)):
            return -((1 << bl) - val)

    return val

def int2bytes(n, byteorder='little', size=None, signed=False):
    """ replicate int.to_bytes from python3 """
    if byteorder not in ('little', 'big'):
        raise ValueError("byteorder must be either 'little' or 'big'")

    bl = n.bit_length()
    if not signed and n < 0:
        raise OverflowError("can't convert negative int to unsigned")
    if signed:
        bl += 1

    rsize = (bl + 7) // 8
    if signed and n < 0:
        n = (1 << (rsize * 8)) + n

    if size is None:
        size = rsize
    elif size < rsize:
        raise OverflowError("int too big to convert")

    buf = bytearray(size)

    if byteorder == 'big':
        it = xrange(size - 1, -1, -1)
    else:
        it = xrange(size)

    for i in it:
        buf[i] = n & 0xff
        n >>= 8
    return bytes(buf)