[Doitsu]

You'll have to use decode():

Spoiler:

def run(bk):
    try:
        px2em = Px2Em()
        for id_file, href in bk.css_iter():
            if bk.launcher_version() <= 20151001: # Sigil-0.8.901 
                data = bk.readfile(id_file).decode('utf-8')
            else:
                data = bk.readfile(id_file)
            px2em._read(data)
            px2em._replace()

            px2em._update(bk, id_file)
    except:
        print(str(sys.exc_info()[0]) + " : " + str(sys.exc_info()[1]))

    return 0