add type annotations in two places where I found this helpful to understand the code

Author: FelixSchwarz

Diff for: schwarz/mailqueue/mailflow_check.py

@@ -14,7 +14,7 @@
 
 __all__ = ['build_check_message', 'send_test_message']
 
-def build_check_message(recipient, sender=None):
+def build_check_message(recipient, sender=None) -> Message:
     mail = Message()
     sender = sender or recipient
     mail['From'] = sender

Diff for: schwarz/mailqueue/message_handler.py

@@ -164,8 +164,8 @@ def retries(self, value):
 
 
 class InMemoryMsg(BaseMsg):
-    def __init__(self, from_addr, to_addrs, msg_bytes):
-        msg_fp = BytesIO(msg_as_bytes(msg_bytes))
+    def __init__(self, from_addr, to_addrs, msg_bytes: bytes):
+        msg_fp = BytesIO(msg_bytes)
         msg = MsgInfo(from_addr, to_addrs, msg_fp)
         super().__init__(msg=msg)