Let's deprecate the pymongo.message module:
https://pymongo.readthedocs.io/en/stable/api/pymongo/message.html

We should remove pymongo.message in 4.0. The motivation is that:

• maintaining these API is harder when we need to keep backwards compatibility.
• applications likely never use this module anyway.
• the module only supports legacy MongoDB wire protocol messages which are no longer used in the majority of cases.