Handle data types of data sent to the server more explicitely

From time to time it happens that the client sends data with bad type to the server (from various reasons) (like in #7 (closed) ). Let's think about a way how to better keep consistency to both help the programmer of this app and not to break the server so often.

Possible hints:

• https://docs.python.org/3/library/typing.html
• https://json-schema.org/

FYI @vsraier