db/message: accept attachments of type pgp-encrypted

In case of a '.gpg' file being attached to a non-encrypted email some
email clients (like thunderbird) will mark this attachment as content
type 'application/pgp-encrypted'. Alot used to discard that mime part
because the same content type is being used in PGP/MIME to signal de
version:
https://tools.ietf.org/html/rfc3156#section-6.1

Now alot differenciates two cases:

* Normal PGP/MIME encrypted messages, where we need to discard the
  'application/pgp-encrypted' mime part. Detected because this part is
  followed by a 'application/octet-stream' mime part.
* A mime part with aplication/pgp-encrypted as content type, that is
  displayed as attachment.

alot/db/message.py

@@ -252,10 +252,13 @@ def get_attachments(self):
                 if ct in ['octet/stream', 'application/octet-stream']:
                     content = part.get_payload(decode=True)
                     ct = helper.guess_mimetype(content)
+                    if (self._attachments and
+                            self._attachments[-1].get_content_type() ==
+                            'application/pgp-encrypted'):
+                        self._attachments.pop()
 
                 if cd.lower().startswith('attachment'):
-                    if ct.lower() not in ['application/pgp-encrypted',
-                                          'application/pgp-signature']:
+                    if ct.lower() not in ['application/pgp-signature']:
                         self._attachments.append(Attachment(part))
                 elif cd.lower().startswith('inline'):
                     if (filename is not None and