str object has no attribute decode Python 3 error

Question

Code:

import imaplib
from email.parser import HeaderParser

conn = imaplib.IMAP4_SSL('imap.gmail.com')
conn.login('example@gmail.com', 'password')
conn.select()
conn.search(None, 'ALL')
data = conn.fetch('1', '(BODY[HEADER])')
header_data = data[1][0][1].decode('utf-8')

Here I am getting an error:

AttributeError: 'str' object has no attribute 'decode'

How can I fix this?