IMAP.py: server responses are in bytes, not string

Following error is seen when parsing server responses for sent mail:

2020-10-12 08:19:11 WARNING: Can't parse FETCH response, we awaited string: b' UID 26855)'
2020-10-12 08:19:11 WARNING: savemessage: Searching mails for new Message-ID failed. Could not determine new UID on Sent.

The comparison with 'type("")' means comparing with 'string' type in Python
3, but the left-hand side is a bytes object.

In case a tuple was received (first case in the code), the input is already
decoded from bytes to strings, but in case a single input was received it
was not.

Note that the comparison with 'type("")' is a bit odd, a more logical way
seems to be:
    if isinstance(item, bytes)

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>

offlineimap/folder/IMAP.py

@@ -482,7 +482,8 @@ class IMAPFolder(BaseFolder):
                              item[1], flags=re.IGNORECASE):
                     found = item[0]
             elif found is not None:
-                if type(item) == type(""):
+                if type(item) == type(b""):
+                    item = item.decode('utf-8')
                     uid = re.search("UID\s+(\d+)", item, flags=re.IGNORECASE)
                     if uid:
                         return int(uid.group(1))