Return (bom_encoding, input), with any BOM removed from the input.

def _detect_bom(input):
    """Return (bom_encoding, input), with any BOM removed from the input."""
    if input.startswith(b'\xFF\xFE'):
        return _UTF16LE, input[2:]
    if input.startswith(b'\xFE\xFF'):
        return _UTF16BE, input[2:]
    if input.startswith(b'\xEF\xBB\xBF'):
        return UTF8, input[3:]
    return None, input