compatibility with dnspython 2.0

Accessing module attributes with _underscore names was a bad idea from
the very beginning so this commit removes assert depending on this. IMHO
it is not worth the trouble.

tools/answer_checker.py

@@ -14,7 +14,6 @@ import pydnstest.mock_client
 
 def unset_flag(message: dns.message.Message, flag: int) -> dns.message.Message:
     """Unsets given flag in given DNS message."""
-    assert flag in dns.flags._by_value, "This is not a DNS flag"  # pylint: disable=W0212
     message.flags &= ~flag
     return message