Accept encrypted messages that use hcp_minimal

in draft-ietf-lamps-header-protection-22, hcp_minimal recommends
"[...]" as the obscured Subject header.  In the pending draft
-23 (hopefully released this week, going into a working group last
call), the same HCP will be renamed to hcp_baseline, but it still
recommends the use of "[...]" for the obscured Subject header.

chatmaild/src/chatmaild/filtermail.py

@@ -111,7 +111,7 @@ def check_encrypted(message):
     """
     if not message.is_multipart():
         return False
-    if message.get("subject") != "...":
+    if message.get("subject") not in {"...", "[...]"}:
         return False
     if message.get_content_type() != "multipart/encrypted":
         return False