We need to make sure that incoming mail via roundup-mailgw works with
Python 3, for each different way documented for it to receive mail.

I have checked in a patch to fix strings/bytes issues for the various
forms of input to roundup-mailgw.

Piped input and mailbox files have been tested.  I have not tested POP
and IMAP input; those need testing to verify they work properly with
both Python 2 and Python 3 after my fixes.

As part of: issue2551008 I am adding some bs2b calls in the 
roundup/anypy/email_.py in decode_header to make sure that it always 
returns byte strings.

All returns of tuples get a bs2b around the first (header value) 
element.

One place I am not sure about is this code:

     if hasattr(header, '_chunks'):                                             
+        # FIXME do we need bs2b around _charset??                              
         return [(_charset._encode(string, str(charset)), str(charset))         
                     for string, charset in header._chunks]   

do we need a bs2b around the return of _charset? AFAICT there is no
test code that exercises this code.

Ezio came up with a different fix that prevents thrashing data from 
encoded to decoded to only encode later.

So no bs2b changes. We still have no test code for the

      if hasattr(header, '_chunks'):                                             

code path.