Fix utf8foldernames configuration option to work with Python3

The current code for supporting the utf8foldernames does not work with
Python3 due to changes in the 'codecs' module and related functions.

This fix adapts the existing code to work with Python3.

Signed-off-by: Amit Ramon <amit@riseup.net>

offlineimap/imaputil.py

@@ -18,6 +18,7 @@
 import re
 import binascii
 import codecs
+from typing import Tuple
 from offlineimap.ui import getglobalui
 
 # Globals
@@ -352,42 +353,45 @@ def decode_mailbox_name(name):
 
 def IMAP_utf8(foldername):
     """Convert IMAP4_utf_7 encoded string to utf-8"""
-    return foldername.decode('imap4-utf-7').encode('utf-8')
+    return codecs.decode(
+        foldername.encode(),
+        'imap4-utf-7'
+    ).encode('utf-8').decode()
 
 
 def utf8_IMAP(foldername):
     """Convert utf-8 encoded string to IMAP4_utf_7"""
-    return foldername.decode('utf-8').encode('imap4-utf-7')
+    return codecs.decode(
+        foldername.encode(),
+        'utf-8'
+    ).encode('imap4-utf-7').decode()
 
 
 # Codec definition
-
 def modified_base64(s):
     s = s.encode('utf-16be')
-    return binascii.b2a_base64(s).rstrip('\n=').replace('/', ',')
+    return binascii.b2a_base64(s).rstrip(b'\n=').replace(b'/', b',')
 
 
 def doB64(_in, r):
     if _in:
-        r.append('&%s-' % modified_base64(''.join(_in)))
+        r.append(b'&%s-' % modified_base64(''.join(_in)))
         del _in[:]
 
 
-def encoder(s):
+def utf7m_encode(text: str) -> Tuple[bytes, int]:
     r = []
     _in = []
-    for c in s:
-        ordC = ord(c)
-        if 0x20 <= ordC <= 0x25 or 0x27 <= ordC <= 0x7e:
+
+    for c in text:
+        if 0x20 <= ord(c) <= 0x7e:
             doB64(_in, r)
-            r.append(c)
-        elif c == '&':
-            doB64(_in, r)
-            r.append('&-')
+            r.append(b'&-' if c == '&' else c.encode())
         else:
             _in.append(c)
+
     doB64(_in, r)
-    return str(''.join(r)), len(s)
+    return b''.join(r), len(text)
 
 
 # decoding
@@ -396,10 +400,10 @@ def modified_unbase64(s):
     return str(b, 'utf-16be')
 
 
-def decoder(s):
+def utf7m_decode(binary: bytes) -> Tuple[str, int]:
     r = []
     decode = []
-    for c in s:
+    for c in binary.decode():
         if c == '&' and not decode:
             decode.append('&')
         elif c == '-' and decode:
@@ -415,26 +419,31 @@ def decoder(s):
 
     if decode:
         r.append(modified_unbase64(''.join(decode[1:])))
-    bin_str = ''.join(r)
-    return bin_str, len(s)
+
+    return ''.join(r), len(binary)
 
 
 class StreamReader(codecs.StreamReader):
     def decode(self, s, errors='strict'):
-        return decoder(s)
+        return utf7m_decode(s)
 
 
 class StreamWriter(codecs.StreamWriter):
     def decode(self, s, errors='strict'):
-        return encoder(s)
+        return utf7m_encode(s)
 
 
-def imap4_utf_7(name):
-    if name == 'imap4-utf-7':
-        return encoder, decoder, StreamReader, StreamWriter
+def utf7m_search_function(name):
+    return codecs.CodecInfo(
+        utf7m_encode,
+        utf7m_decode,
+        StreamReader,
+        StreamWriter,
+        name='imap4-utf-7'
+    )
 
 
-codecs.register(imap4_utf_7)
+codecs.register(utf7m_search_function)
 
 
 def foldername_to_imapname(folder_name):