Extracting the body of an email from mbox file, decoding it to plain text regardless of Charset and Content Transfer Encoding

I am trying to use Python 3 to extract the body of email messages from a thunderbird mbox file. It is an IMAP account.

I would like to have the text part of the body of the email available to process as a unicode string. It should 'look like' the email does in Thunderbird, and not contain escaped characters such as \r\n =20 etc.

I think that it is the Content Transfer Encodings that I don't know how to decode or remove. I receive emails with a variety of different Content Types, and different Content Transfer Encodings. This is my current attempt :

import mailbox
import quopri,base64

def myconvert(encoded,ContentTransferEncoding):
    if ContentTransferEncoding == 'quoted-printable':
        result = quopri.decodestring(encoded)
    elif ContentTransferEncoding == 'base64':
        result = base64.b64decode(encoded)

mboxfile = 'C:/Users/Username/Documents/Thunderbird/Data/profile/ImapMail/server.name/INBOX'

for msg in mailbox.mbox(mboxfile):
    if msg.is_multipart():    #Walk through the parts of the email to find the text body.
        for part in msg.walk():
            if part.is_multipart(): # If part is multipart, walk through the subparts.
                for subpart in part.walk():
                    if subpart.get_content_type() == 'text/plain':
                        body = subpart.get_payload() # Get the subpart payload (i.e the message body)
                    for k,v in subpart.items():
                            if k == 'Content-Transfer-Encoding':
                                cte = v             # Keep the Content Transfer Encoding
            elif subpart.get_content_type() == 'text/plain':
                body = part.get_payload()           # part isn't multipart Get the payload
                for k,v in part.items():
                    if k == 'Content-Transfer-Encoding':
                        cte = v                      # Keep the Content Transfer Encoding

print(body)
print('Body is of type:',ty