Message.sender can now be None if not available

This was a change applied to the upstream Bot API some time ago, but
wasn't reflected in botogram's source code. This commit marks that
field as optional so that the deserialization of objects won't fail
anymore for channel messages.

Author: Pietro Albini

botogram/objects/messages.py

@@ -30,11 +30,11 @@ def from_(self):
 
     required = {
         "message_id": int,
-        "from": User,
         "date": int,
         "chat": Chat,
     }
     optional = {
+        "from": User,
         "forward_from": User,
         "forward_date": int,
         "reply_to_message": _itself,